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Onconova Therapeutics, Inc. – ‘10-Q’ for 3/31/20 – ‘EX-101.LAB’

On:  Friday, 5/15/20, at 4:11pm ET   ·   For:  3/31/20   ·   Accession #:  1104659-20-62608   ·   File #:  1-36020

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  As Of               Filer                 Filing    For·On·As Docs:Size             Issuer                      Filing Agent

 5/15/20  Onconova Therapeutics, Inc.       10-Q        3/31/20   64:5M                                     Toppan Merrill/FA

Quarterly Report   —   Form 10-Q   —   Sect. 13 / 15(d) – SEA’34
Filing Table of Contents

Document/Exhibit                   Description                      Pages   Size 

 1: 10-Q        Quarterly Report                                    HTML    328K 
 2: EX-31.1     Certification -- §302 - SOA'02                      HTML     23K 
 3: EX-31.2     Certification -- §302 - SOA'02                      HTML     23K 
 4: EX-32.1     Certification -- §906 - SOA'02                      HTML     20K 
 5: EX-32.2     Certification -- §906 - SOA'02                      HTML     20K 
30: R1          Document and Entity Information                     HTML     48K 
59: R2          Condensed Consolidated Balance Sheets               HTML     90K 
42: R3          Condensed Consolidated Balance Sheets               HTML     38K 
                (Parenthetical)                                                  
21: R4          Condensed Consolidated Statements of Operations     HTML     48K 
31: R5          Condensed Consolidated Statements of Comprehensive  HTML     35K 
                Loss                                                             
60: R6          Consolidated Statement of Stockholders' Equity      HTML     56K 
                (Deficit)                                                        
43: R7          Condensed Consolidated Statements of Cash Flows     HTML     69K 
22: R8          Nature of Business                                  HTML     41K 
29: R9          Summary of Significant Accounting Policies          HTML     55K 
52: R10         Revenue                                             HTML     50K 
58: R11         Net Loss Per Share of Common Stock                  HTML     33K 
36: R12         Warrants                                            HTML    184K 
14: R13         Balance Sheet Detail                                HTML     82K 
51: R14         Fair Value Measurements                             HTML     88K 
57: R15         Stock-Based Compensation                            HTML    154K 
35: R16         Research Agreements                                 HTML     22K 
13: R17         License and Collaboration Agreements                HTML     32K 
53: R18         Related-Party Transactions                          HTML     24K 
56: R19         Securities Registrations and Sales Agreements       HTML     25K 
34: R20         Summary of Significant Accounting Policies          HTML     65K 
                (Policies)                                                       
24: R21         Revenue (Tables)                                    HTML     51K 
48: R22         Net Loss Per Share of Common Stock (Tables)         HTML     33K 
64: R23         Warrants (Tables)                                   HTML    185K 
33: R24         Balance Sheet Detail (Tables)                       HTML     85K 
23: R25         Fair Value Measurements (Tables)                    HTML     82K 
47: R26         Stock-Based Compensation (Tables)                   HTML    149K 
63: R27         Nature of Business - Liquidity (Details)            HTML     79K 
32: R28         Summary of Significant Accounting Policies -        HTML     21K 
                Segment Information (Details)                                    
25: R29         Summary of Significant Accounting Policies -        HTML     29K 
                Leases (Details)                                                 
16: R30         Revenue (Details)                                   HTML     38K 
37: R31         Net Loss Per Share of Common Stock (Details)        HTML     27K 
54: R32         Warrants (Details)                                  HTML    131K 
49: R33         Balance Sheet Detail - Prepaid Expenses and Other   HTML     33K 
                Current Assets (Details)                                         
17: R34         Balance Sheet Detail Property and Equipment         HTML     28K 
                (Details)                                                        
38: R35         Balance Sheet Detail - Accrued Expenses and Other   HTML     29K 
                Current Liabilities (Details)                                    
55: R36         Fair Value Measurements (Details)                   HTML     34K 
50: R37         Fair Value Measurements - Rights Offering           HTML     42K 
                (Details)                                                        
15: R38         Fair Value Measurements - Weighted-average          HTML     30K 
                assumptions (Details)                                            
39: R39         Fair Value Measurements - Fair Value Hierarchy      HTML     30K 
                Table (Details)                                                  
19: R40         Fair Value Measurements - Transfers (Details)       HTML     28K 
26: R41         Stock-Based Compensation (Details)                  HTML     41K 
61: R42         Stock-Based Compensation - Expense (Details)        HTML     29K 
44: R43         Stock-Based Compensation - Activity (Details)       HTML     78K 
20: R44         Stock-Based Compensation - Outstanding and          HTML     62K 
                Exercisable (Details)                                            
27: R45         Stock-Based Compensation - Unrecognized             HTML     25K 
                Compensation Expense (Details)                                   
62: R46         Stock-Based Compensation - Fair Value Assumptions   HTML     37K 
                (Details)                                                        
45: R47         Research Agreements (Details)                       HTML     23K 
18: R48         License and Collaboration Agreements (Details)      HTML     68K 
28: R49         Related-Party Transactions (Details)                HTML     30K 
41: R50         Securities Registrations and Sales Agreements -     HTML     49K 
                Underwriting agreement with HCW (Details)                        
40: XML         IDEA XML File -- Filing Summary                      XML    111K 
12: EXCEL       IDEA Workbook of Financial Reports                  XLSX     65K 
 6: EX-101.INS  XBRL Instance -- ontx-20200331                       XML   1.58M 
 8: EX-101.CAL  XBRL Calculations -- ontx-20200331_cal               XML     98K 
 9: EX-101.DEF  XBRL Definitions -- ontx-20200331_def                XML    385K 
10: EX-101.LAB  XBRL Labels -- ontx-20200331_lab                     XML    953K 
11: EX-101.PRE  XBRL Presentations -- ontx-20200331_pre              XML    672K 
 7: EX-101.SCH  XBRL Schema -- ontx-20200331                         XSD    137K 
46: ZIP         XBRL Zipped Folder -- 0001104659-20-062608-xbrl      Zip    113K

‘EX-101.LAB’   —   XBRL Labels — ontx-20200331_lab

This Exhibit is an XBRL XML File.

                                                                                                                                                                                
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<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_DocumentType" xlink:label="dei_DocumentType" xmlns="http://www.xbrl.org/2003/linkbase"/>
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<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_DocumentPeriodEndDate" xlink:label="dei_DocumentPeriodEndDate" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="dei_DocumentPeriodEndDate" xlink:to="dei_DocumentPeriodEndDate_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="dei_DocumentPeriodEndDate_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Document Period End Date </label>
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityRegistrantName" xlink:label="dei_EntityRegistrantName" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="dei_EntityRegistrantName" xlink:to="dei_EntityRegistrantName_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="dei_EntityRegistrantName_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Entity Registrant Name </label>
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityCurrentReportingStatus" xlink:label="dei_EntityCurrentReportingStatus" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="dei_EntityCurrentReportingStatus" xlink:to="dei_EntityCurrentReportingStatus_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="dei_EntityCurrentReportingStatus_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Entity Current Reporting Status </label>
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityInteractiveDataCurrent" xlink:label="dei_EntityInteractiveDataCurrent" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="dei_EntityInteractiveDataCurrent" xlink:to="dei_EntityInteractiveDataCurrent_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="dei_EntityInteractiveDataCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Entity Interactive Data Current </label>
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityFilerCategory" xlink:label="dei_EntityFilerCategory" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="dei_EntityFilerCategory" xlink:to="dei_EntityFilerCategory_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="dei_EntityFilerCategory_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Entity Filer Category </label>
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntitySmallBusiness" xlink:label="dei_EntitySmallBusiness" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="dei_EntitySmallBusiness" xlink:to="dei_EntitySmallBusiness_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="dei_EntitySmallBusiness_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Entity Small Business </label>
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityEmergingGrowthCompany" xlink:label="dei_EntityEmergingGrowthCompany" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="dei_EntityEmergingGrowthCompany" xlink:to="dei_EntityEmergingGrowthCompany_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="dei_EntityEmergingGrowthCompany_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Entity Emerging Growth Company </label>
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityShellCompany" xlink:label="dei_EntityShellCompany" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="dei_EntityShellCompany" xlink:to="dei_EntityShellCompany_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="dei_EntityShellCompany_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Entity Shell Company </label>
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityCommonStockSharesOutstanding" xlink:label="dei_EntityCommonStockSharesOutstanding" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="dei_EntityCommonStockSharesOutstanding" xlink:to="dei_EntityCommonStockSharesOutstanding_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="dei_EntityCommonStockSharesOutstanding_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Entity Common Stock, Shares Outstanding </label>
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityCentralIndexKey" xlink:label="dei_EntityCentralIndexKey" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="dei_EntityCentralIndexKey" xlink:to="dei_EntityCentralIndexKey_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="dei_EntityCentralIndexKey_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Entity Central Index Key </label>
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_CurrentFiscalYearEndDate" xlink:label="dei_CurrentFiscalYearEndDate" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="dei_CurrentFiscalYearEndDate" xlink:to="dei_CurrentFiscalYearEndDate_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="dei_CurrentFiscalYearEndDate_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Current Fiscal Year End Date </label>
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_DocumentFiscalYearFocus" xlink:label="dei_DocumentFiscalYearFocus" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="dei_DocumentFiscalYearFocus" xlink:to="dei_DocumentFiscalYearFocus_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="dei_DocumentFiscalYearFocus_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Document Fiscal Year Focus </label>
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_DocumentFiscalPeriodFocus" xlink:label="dei_DocumentFiscalPeriodFocus" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="dei_DocumentFiscalPeriodFocus" xlink:to="dei_DocumentFiscalPeriodFocus_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="dei_DocumentFiscalPeriodFocus_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Document Fiscal Period Focus </label>
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_AmendmentFlag" xlink:label="dei_AmendmentFlag" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="dei_AmendmentFlag" xlink:to="dei_AmendmentFlag_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="dei_AmendmentFlag_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Amendment Flag </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StatementOfFinancialPositionAbstract" xlink:label="us-gaap_StatementOfFinancialPositionAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_StatementOfFinancialPositionAbstract" xlink:to="us-gaap_StatementOfFinancialPositionAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_StatementOfFinancialPositionAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Condensed Consolidated Balance Sheets </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AssetsAbstract" xlink:label="us-gaap_AssetsAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_AssetsAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AssetsAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Assets [Abstract] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_AssetsAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Assets </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AssetsCurrentAbstract" xlink:label="us-gaap_AssetsCurrentAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_AssetsCurrentAbstract" xlink:to="us-gaap_AssetsCurrentAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AssetsCurrentAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Assets, Current [Abstract] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_AssetsCurrentAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Current assets: </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents" xlink:label="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents" xlink:to="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel" xlink:label="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Cash and cash equivalents at end of period </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/periodStartLabel" xlink:label="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Cash and cash equivalents at beginning of period </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Cash and cash equivalents </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ReceivablesNetCurrent" xlink:label="us-gaap_ReceivablesNetCurrent" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ReceivablesNetCurrent" xlink:to="us-gaap_ReceivablesNetCurrent_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ReceivablesNetCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Receivables, Net, Current </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ReceivablesNetCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Receivables </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PrepaidExpenseAndOtherAssetsCurrent" xlink:label="us-gaap_PrepaidExpenseAndOtherAssetsCurrent" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_PrepaidExpenseAndOtherAssetsCurrent" xlink:to="us-gaap_PrepaidExpenseAndOtherAssetsCurrent_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_PrepaidExpenseAndOtherAssetsCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Prepaid Expense and Other Assets, Current </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_PrepaidExpenseAndOtherAssetsCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Prepaid expenses and other current assets </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_PrepaidExpenseAndOtherAssetsCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Prepaid expenses and other current assets </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AssetsCurrent" xlink:label="us-gaap_AssetsCurrent" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_AssetsCurrent" xlink:to="us-gaap_AssetsCurrent_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AssetsCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Assets, Current </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_AssetsCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Total current assets </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:label="us-gaap_PropertyPlantAndEquipmentNet" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_PropertyPlantAndEquipmentNet" xlink:to="us-gaap_PropertyPlantAndEquipmentNet_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_PropertyPlantAndEquipmentNet_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Property, Plant and Equipment, Net </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_PropertyPlantAndEquipmentNet_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Property and equipment, net </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_PropertyPlantAndEquipmentNet_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Property and equipment, net </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OtherAssetsNoncurrent" xlink:label="us-gaap_OtherAssetsNoncurrent" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OtherAssetsNoncurrent" xlink:to="us-gaap_OtherAssetsNoncurrent_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OtherAssetsNoncurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Other Assets, Noncurrent </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_OtherAssetsNoncurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Other non-current assets </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_Assets" xlink:label="us-gaap_Assets" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_Assets" xlink:to="us-gaap_Assets_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_Assets_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Assets </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_Assets_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Total assets </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:label="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_LiabilitiesAndStockholdersEquityAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_LiabilitiesAndStockholdersEquityAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Liabilities and Equity [Abstract] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_LiabilitiesAndStockholdersEquityAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Liabilities and stockholders' equity </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_LiabilitiesCurrentAbstract" xlink:label="us-gaap_LiabilitiesCurrentAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_LiabilitiesCurrentAbstract" xlink:to="us-gaap_LiabilitiesCurrentAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_LiabilitiesCurrentAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Liabilities, Current [Abstract] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_LiabilitiesCurrentAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Current liabilities: </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AccountsPayableCurrent" xlink:label="us-gaap_AccountsPayableCurrent" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_AccountsPayableCurrent" xlink:to="us-gaap_AccountsPayableCurrent_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AccountsPayableCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Accounts Payable, Current </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_AccountsPayableCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Accounts payable </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AccruedLiabilitiesCurrent" xlink:label="us-gaap_AccruedLiabilitiesCurrent" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_AccruedLiabilitiesCurrent" xlink:to="us-gaap_AccruedLiabilitiesCurrent_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AccruedLiabilitiesCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Accrued Expenses and Other Current Liabilities, Current </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_AccruedLiabilitiesCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Accrued expenses and other current liabilities </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_AccruedLiabilitiesCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Accrued expenses and other current liabilities: </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ContractWithCustomerLiabilityCurrent" xlink:label="us-gaap_ContractWithCustomerLiabilityCurrent" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ContractWithCustomerLiabilityCurrent" xlink:to="us-gaap_ContractWithCustomerLiabilityCurrent_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ContractWithCustomerLiabilityCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Contract with Customer, Liability, Current </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel" xlink:label="us-gaap_ContractWithCustomerLiabilityCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Balance at the end of the period </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/periodStartLabel" xlink:label="us-gaap_ContractWithCustomerLiabilityCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Balance at the beginning of the period </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ContractWithCustomerLiabilityCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Deferred revenue </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_LiabilitiesCurrent" xlink:label="us-gaap_LiabilitiesCurrent" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_LiabilitiesCurrent" xlink:to="us-gaap_LiabilitiesCurrent_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_LiabilitiesCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Liabilities, Current </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_LiabilitiesCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Total current liabilities </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_WarrantsAndRightsOutstanding" xlink:label="us-gaap_WarrantsAndRightsOutstanding" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_WarrantsAndRightsOutstanding" xlink:to="us-gaap_WarrantsAndRightsOutstanding_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_WarrantsAndRightsOutstanding_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Warrants and Rights Outstanding </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_WarrantsAndRightsOutstanding_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Warrant liability </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ContractWithCustomerLiabilityNoncurrent" xlink:label="us-gaap_ContractWithCustomerLiabilityNoncurrent" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ContractWithCustomerLiabilityNoncurrent" xlink:to="us-gaap_ContractWithCustomerLiabilityNoncurrent_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ContractWithCustomerLiabilityNoncurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Contract with Customer, Liability, Noncurrent </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ContractWithCustomerLiabilityNoncurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Deferred revenue, non-current </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_Liabilities" xlink:label="us-gaap_Liabilities" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_Liabilities" xlink:to="us-gaap_Liabilities_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_Liabilities_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Liabilities </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_Liabilities_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Total liabilities </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CommitmentsAndContingencies" xlink:label="us-gaap_CommitmentsAndContingencies" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_CommitmentsAndContingencies" xlink:to="us-gaap_CommitmentsAndContingencies_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_CommitmentsAndContingencies_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Commitments and Contingencies. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_CommitmentsAndContingencies_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Commitments and contingencies </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract" xlink:label="us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract" xlink:to="us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest [Abstract] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Stockholders' equity: </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PreferredStockValue" xlink:label="us-gaap_PreferredStockValue" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_PreferredStockValue" xlink:to="us-gaap_PreferredStockValue_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_PreferredStockValue_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Preferred Stock, Value, Issued </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_PreferredStockValue_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Preferred stock, $0.01 par value, 5,000,000 authorized at March 31, 2020 and December 31, 2019, none issued and outstanding at March 31, 2020 and December 31, 2019 </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CommonStockValue" xlink:label="us-gaap_CommonStockValue" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_CommonStockValue" xlink:to="us-gaap_CommonStockValue_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_CommonStockValue_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Common Stock, Value, Issued </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:label="us-gaap_CommonStockValue_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Common stock, $0.01 par value, 250,000,000 authorized at March 31, 2020 and December 31, 2019, 167,416,070 and 111,167,352 shares issued and outstanding at March 31, 2020 and December 31, 2019 </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AdditionalPaidInCapitalCommonStock" xlink:label="us-gaap_AdditionalPaidInCapitalCommonStock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_AdditionalPaidInCapitalCommonStock" xlink:to="us-gaap_AdditionalPaidInCapitalCommonStock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AdditionalPaidInCapitalCommonStock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Additional Paid in Capital, Common Stock </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_AdditionalPaidInCapitalCommonStock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Additional paid in capital </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax" xlink:label="us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax" xlink:to="us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Accumulated Other Comprehensive Income (Loss), Net of Tax </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Accumulated other comprehensive loss </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RetainedEarningsAccumulatedDeficit" xlink:label="us-gaap_RetainedEarningsAccumulatedDeficit" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_RetainedEarningsAccumulatedDeficit" xlink:to="us-gaap_RetainedEarningsAccumulatedDeficit_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_RetainedEarningsAccumulatedDeficit_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Retained Earnings (Accumulated Deficit) </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2009/role/negatedLabel" xlink:label="us-gaap_RetainedEarningsAccumulatedDeficit_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Accumulated deficit </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_RetainedEarningsAccumulatedDeficit_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Accumulated deficit </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StockholdersEquity" xlink:label="us-gaap_StockholdersEquity" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_StockholdersEquity" xlink:to="us-gaap_StockholdersEquity_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_StockholdersEquity_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Stockholders' Equity Attributable to Parent </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_StockholdersEquity_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Total Onconova Therapeutics, Inc. stockholders' equity </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" xlink:label="us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" xlink:to="us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel" xlink:label="us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Balance </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/periodStartLabel" xlink:label="us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Balance </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Total stockholders' equity </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_LiabilitiesAndStockholdersEquity" xlink:label="us-gaap_LiabilitiesAndStockholdersEquity" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_LiabilitiesAndStockholdersEquity" xlink:to="us-gaap_LiabilitiesAndStockholdersEquity_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_LiabilitiesAndStockholdersEquity_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Liabilities and Equity </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_LiabilitiesAndStockholdersEquity_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Total liabilities and stockholders' equity </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PreferredStockParOrStatedValuePerShare" xlink:label="us-gaap_PreferredStockParOrStatedValuePerShare" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_PreferredStockParOrStatedValuePerShare" xlink:to="us-gaap_PreferredStockParOrStatedValuePerShare_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_PreferredStockParOrStatedValuePerShare_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Preferred Stock, Par or Stated Value Per Share </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_PreferredStockParOrStatedValuePerShare_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Preferred stock, par value (in dollars per share) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PreferredStockSharesAuthorized" xlink:label="us-gaap_PreferredStockSharesAuthorized" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_PreferredStockSharesAuthorized" xlink:to="us-gaap_PreferredStockSharesAuthorized_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_PreferredStockSharesAuthorized_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Preferred Stock, Shares Authorized </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_PreferredStockSharesAuthorized_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Preferred stock, shares authorized </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PreferredStockSharesIssued" xlink:label="us-gaap_PreferredStockSharesIssued" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_PreferredStockSharesIssued" xlink:to="us-gaap_PreferredStockSharesIssued_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_PreferredStockSharesIssued_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Preferred Stock, Shares Issued </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_PreferredStockSharesIssued_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Preferred stock, shares issued </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PreferredStockSharesOutstanding" xlink:label="us-gaap_PreferredStockSharesOutstanding" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_PreferredStockSharesOutstanding" xlink:to="us-gaap_PreferredStockSharesOutstanding_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_PreferredStockSharesOutstanding_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Preferred Stock, Shares Outstanding </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_PreferredStockSharesOutstanding_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Preferred stock, shares outstanding </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CommonStockParOrStatedValuePerShare" xlink:label="us-gaap_CommonStockParOrStatedValuePerShare" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_CommonStockParOrStatedValuePerShare" xlink:to="us-gaap_CommonStockParOrStatedValuePerShare_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_CommonStockParOrStatedValuePerShare_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Common Stock, Par or Stated Value Per Share </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_CommonStockParOrStatedValuePerShare_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Common stock, par value (in dollars per share) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CommonStockSharesAuthorized" xlink:label="us-gaap_CommonStockSharesAuthorized" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_CommonStockSharesAuthorized" xlink:to="us-gaap_CommonStockSharesAuthorized_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_CommonStockSharesAuthorized_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Common Stock, Shares Authorized </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_CommonStockSharesAuthorized_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Common Stock authorized (in shares) </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:label="us-gaap_CommonStockSharesAuthorized_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Common stock, shares authorized </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CommonStockSharesIssued" xlink:label="us-gaap_CommonStockSharesIssued" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_CommonStockSharesIssued" xlink:to="us-gaap_CommonStockSharesIssued_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_CommonStockSharesIssued_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Common Stock, Shares, Issued </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel" xlink:label="us-gaap_CommonStockSharesIssued_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Balance (in shares) </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/periodStartLabel" xlink:label="us-gaap_CommonStockSharesIssued_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Balance (in shares) </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_CommonStockSharesIssued_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Common stock, shares issued </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CommonStockSharesOutstanding" xlink:label="us-gaap_CommonStockSharesOutstanding" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_CommonStockSharesOutstanding" xlink:to="us-gaap_CommonStockSharesOutstanding_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_CommonStockSharesOutstanding_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Common Stock, Shares, Outstanding </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_CommonStockSharesOutstanding_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Common stock, shares outstanding </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:label="us-gaap_CommonStockSharesOutstanding_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Common stock outstanding (in shares) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IncomeStatementAbstract" xlink:label="us-gaap_IncomeStatementAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_IncomeStatementAbstract" xlink:to="us-gaap_IncomeStatementAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_IncomeStatementAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Condensed Consolidated Statements of Operations </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_Revenues" xlink:label="us-gaap_Revenues" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_Revenues" xlink:to="us-gaap_Revenues_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_Revenues_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Revenue </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_Revenues_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Revenue </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OperatingExpensesAbstract" xlink:label="us-gaap_OperatingExpensesAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OperatingExpensesAbstract" xlink:to="us-gaap_OperatingExpensesAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OperatingExpensesAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Operating Expenses [Abstract] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_OperatingExpensesAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Operating expenses: </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_GeneralAndAdministrativeExpense" xlink:label="us-gaap_GeneralAndAdministrativeExpense" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_GeneralAndAdministrativeExpense" xlink:to="us-gaap_GeneralAndAdministrativeExpense_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_GeneralAndAdministrativeExpense_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> General and Administrative Expense </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_GeneralAndAdministrativeExpense_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> General and administrative </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ResearchAndDevelopmentExpense" xlink:label="us-gaap_ResearchAndDevelopmentExpense" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ResearchAndDevelopmentExpense" xlink:to="us-gaap_ResearchAndDevelopmentExpense_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ResearchAndDevelopmentExpense_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Research and Development Expense </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ResearchAndDevelopmentExpense_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Research and development </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OperatingExpenses" xlink:label="us-gaap_OperatingExpenses" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OperatingExpenses" xlink:to="us-gaap_OperatingExpenses_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OperatingExpenses_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Operating Expenses </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_OperatingExpenses_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Total operating expenses </label>
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<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OperatingIncomeLoss" xlink:to="us-gaap_OperatingIncomeLoss_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
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<label xlink:type="resource" xlink:role="http://www.xbrl.org/2009/role/negatedLabel" xlink:label="us-gaap_FairValueAdjustmentOfWarrants_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Change in fair value of warrant liability </label>
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<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OtherNonoperatingIncomeExpense" xlink:label="us-gaap_OtherNonoperatingIncomeExpense" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OtherNonoperatingIncomeExpense" xlink:to="us-gaap_OtherNonoperatingIncomeExpense_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OtherNonoperatingIncomeExpense_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Other income, net </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_OtherNonoperatingIncomeExpense_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Other income, net </label>
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<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ProfitLoss" xlink:to="us-gaap_ProfitLoss_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ProfitLoss_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Net Income (Loss), Including Portion Attributable to Noncontrolling Interest </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ProfitLoss_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Net loss </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_ProfitLoss_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Net loss </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="us-gaap_NetIncomeLoss" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_NetIncomeLoss" xlink:to="us-gaap_NetIncomeLoss_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_NetIncomeLoss_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Net Income (Loss) Attributable to Parent </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_NetIncomeLoss_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Net loss attributable to Onconova Therapeutics, Inc. </label>
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<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_EarningsPerShareBasicAndDiluted" xlink:to="us-gaap_EarningsPerShareBasicAndDiluted_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_EarningsPerShareBasicAndDiluted_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Earnings Per Share, Basic and Diluted </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_EarningsPerShareBasicAndDiluted_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Net loss per share, basic and diluted (in dollars per share) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted" xlink:label="us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted" xlink:to="us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Weighted Average Number of Shares Outstanding, Basic and Diluted </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Basic and diluted weighted average shares outstanding (in shares) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StatementOfIncomeAndComprehensiveIncomeAbstract" xlink:label="us-gaap_StatementOfIncomeAndComprehensiveIncomeAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_StatementOfIncomeAndComprehensiveIncomeAbstract" xlink:to="us-gaap_StatementOfIncomeAndComprehensiveIncomeAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_StatementOfIncomeAndComprehensiveIncomeAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Condensed Consolidated Statements of Comprehensive Loss </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax" xlink:label="us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax" xlink:to="us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Foreign currency translation adjustments, net </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTax" xlink:label="us-gaap_OtherComprehensiveIncomeLossNetOfTax" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OtherComprehensiveIncomeLossNetOfTax" xlink:to="us-gaap_OtherComprehensiveIncomeLossNetOfTax_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OtherComprehensiveIncomeLossNetOfTax_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Other Comprehensive Income (Loss), Net of Tax </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_OtherComprehensiveIncomeLossNetOfTax_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Other comprehensive loss, net of tax </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest" xlink:label="us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest" xlink:to="us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Comprehensive loss </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ComprehensiveIncomeNetOfTax" xlink:label="us-gaap_ComprehensiveIncomeNetOfTax" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ComprehensiveIncomeNetOfTax" xlink:to="us-gaap_ComprehensiveIncomeNetOfTax_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ComprehensiveIncomeNetOfTax_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Comprehensive Income (Loss), Net of Tax, Attributable to Parent </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_ComprehensiveIncomeNetOfTax_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Comprehensive loss attributable to Onconova Therapeutics, Inc. </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StatementOfStockholdersEquityAbstract" xlink:label="us-gaap_StatementOfStockholdersEquityAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_StatementOfStockholdersEquityAbstract" xlink:to="us-gaap_StatementOfStockholdersEquityAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_StatementOfStockholdersEquityAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Consolidated Statements of Stockholders' Equity (Deficit) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:label="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Increase (Decrease) in Stockholders' Equity [Roll Forward] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Increase (Decrease) in Stockholders' Equity (Deficit) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent" xlink:label="us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent" xlink:to="us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:label="us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Other comprehensive loss </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue" xlink:label="us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue" xlink:to="us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Stock-based compensation </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues" xlink:label="us-gaap_StockIssuedDuringPeriodValueNewIssues" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_StockIssuedDuringPeriodValueNewIssues" xlink:to="us-gaap_StockIssuedDuringPeriodValueNewIssues_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_StockIssuedDuringPeriodValueNewIssues_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Stock Issued During Period, Value, New Issues </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_StockIssuedDuringPeriodValueNewIssues_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Issuance of common stock, Net </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:label="us-gaap_StockIssuedDuringPeriodValueNewIssues_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Value of common stock </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:label="us-gaap_StockIssuedDuringPeriodSharesNewIssues" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:to="us-gaap_StockIssuedDuringPeriodSharesNewIssues_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_StockIssuedDuringPeriodSharesNewIssues_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Stock Issued During Period, Shares, New Issues </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_StockIssuedDuringPeriodSharesNewIssues_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Issuance of common stock, net (in shares) </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:label="us-gaap_StockIssuedDuringPeriodSharesNewIssues_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Issuance of stock (in shares) </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_StockIssuedDuringPeriodValueWarrantsExercised" xlink:label="ontx_StockIssuedDuringPeriodValueWarrantsExercised" xmlns="http://www.xbrl.org/2003/linkbase"/>
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<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_StockIssuedDuringPeriodValueWarrantsExercised_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Stock Issued During Period, Value, Warrants Exercised </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_StockIssuedDuringPeriodValueWarrantsExercised_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Issuance of common stock upon exercise of warrants </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_StockIssuedDuringPeriodSharesWarrantsExercised" xlink:label="ontx_StockIssuedDuringPeriodSharesWarrantsExercised" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_StockIssuedDuringPeriodSharesWarrantsExercised" xlink:to="ontx_StockIssuedDuringPeriodSharesWarrantsExercised_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_StockIssuedDuringPeriodSharesWarrantsExercised_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Shares of stock issued upon the exercise of warrants. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_StockIssuedDuringPeriodSharesWarrantsExercised_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Stock Issued During Period, Shares, Warrants Exercised </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_StockIssuedDuringPeriodSharesWarrantsExercised_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Issuance of common stock upon exercise of warrants (in shares) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StatementOfCashFlowsAbstract" xlink:label="us-gaap_StatementOfCashFlowsAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_StatementOfCashFlowsAbstract" xlink:to="us-gaap_StatementOfCashFlowsAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_StatementOfCashFlowsAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Condensed Consolidated Statements of Cash Flows </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract" xlink:label="us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Net Cash Provided by (Used in) Operating Activities [Abstract] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Operating activities: </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:label="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Adjustment to reconcile net loss to net cash used in operating activities: </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_DepreciationDepletionAndAmortization" xlink:label="us-gaap_DepreciationDepletionAndAmortization" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_DepreciationDepletionAndAmortization" xlink:to="us-gaap_DepreciationDepletionAndAmortization_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_DepreciationDepletionAndAmortization_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Depreciation, Depletion and Amortization </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_DepreciationDepletionAndAmortization_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Depreciation and amortization </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensation" xlink:label="us-gaap_ShareBasedCompensation" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensation" xlink:to="us-gaap_ShareBasedCompensation_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensation_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShareBasedCompensation_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Stock compensation expense </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:label="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:to="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Increase (Decrease) in Operating Capital [Abstract] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Changes in assets and liabilities: </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IncreaseDecreaseInReceivables" xlink:label="us-gaap_IncreaseDecreaseInReceivables" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_IncreaseDecreaseInReceivables" xlink:to="us-gaap_IncreaseDecreaseInReceivables_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_IncreaseDecreaseInReceivables_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Increase (Decrease) in Receivables </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2009/role/negatedLabel" xlink:label="us-gaap_IncreaseDecreaseInReceivables_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Receivables </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets" xlink:label="us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets" xlink:to="us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Increase (Decrease) in Prepaid Expense and Other Assets </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2009/role/negatedLabel" xlink:label="us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Prepaid expenses and other current assets </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IncreaseDecreaseInAccountsPayable" xlink:label="us-gaap_IncreaseDecreaseInAccountsPayable" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_IncreaseDecreaseInAccountsPayable" xlink:to="us-gaap_IncreaseDecreaseInAccountsPayable_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_IncreaseDecreaseInAccountsPayable_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Increase (Decrease) in Accounts Payable </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_IncreaseDecreaseInAccountsPayable_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Accounts payable </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_IncreaseDecreaseInAccruedExpensesAndOtherLiabilitiesCurrent" xlink:label="ontx_IncreaseDecreaseInAccruedExpensesAndOtherLiabilitiesCurrent" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_IncreaseDecreaseInAccruedExpensesAndOtherLiabilitiesCurrent" xlink:to="ontx_IncreaseDecreaseInAccruedExpensesAndOtherLiabilitiesCurrent_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_IncreaseDecreaseInAccruedExpensesAndOtherLiabilitiesCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> The increase (decrease) during the reporting period in accrued expenses and other current liabilities. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_IncreaseDecreaseInAccruedExpensesAndOtherLiabilitiesCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Increase (Decrease) in Accrued Expenses and Other Liabilities, Current </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_IncreaseDecreaseInAccruedExpensesAndOtherLiabilitiesCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Accrued expenses and other current liabilities </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IncreaseDecreaseInContractWithCustomerLiability" xlink:label="us-gaap_IncreaseDecreaseInContractWithCustomerLiability" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_IncreaseDecreaseInContractWithCustomerLiability" xlink:to="us-gaap_IncreaseDecreaseInContractWithCustomerLiability_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_IncreaseDecreaseInContractWithCustomerLiability_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Increase (Decrease) in Contract With Customer Liability </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_IncreaseDecreaseInContractWithCustomerLiability_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Deferred revenue </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivities" xlink:label="us-gaap_NetCashProvidedByUsedInOperatingActivities" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_NetCashProvidedByUsedInOperatingActivities" xlink:to="us-gaap_NetCashProvidedByUsedInOperatingActivities_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_NetCashProvidedByUsedInOperatingActivities_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Net Cash Provided by (Used in) Operating Activities </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_NetCashProvidedByUsedInOperatingActivities_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Net cash used in operating activities </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:label="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:to="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Net Cash Provided by (Used in) Financing Activities [Abstract] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Financing activities: </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ProceedsFromIssuanceOfCommonStock" xlink:label="us-gaap_ProceedsFromIssuanceOfCommonStock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ProceedsFromIssuanceOfCommonStock" xlink:to="us-gaap_ProceedsFromIssuanceOfCommonStock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ProceedsFromIssuanceOfCommonStock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Proceeds from Issuance of Common Stock </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:label="us-gaap_ProceedsFromIssuanceOfCommonStock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Proceeds from the sale of common stock and warrants, net of costs </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ProceedsFromWarrantExercises" xlink:label="us-gaap_ProceedsFromWarrantExercises" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ProceedsFromWarrantExercises" xlink:to="us-gaap_ProceedsFromWarrantExercises_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ProceedsFromWarrantExercises_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Proceeds from Warrant Exercises </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:label="us-gaap_ProceedsFromWarrantExercises_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Proceeds from the exercise of warrants </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ProceedsFromOtherEquity" xlink:label="us-gaap_ProceedsFromOtherEquity" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ProceedsFromOtherEquity" xlink:to="us-gaap_ProceedsFromOtherEquity_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ProceedsFromOtherEquity_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Proceeds from Pre-funded Warrant Exercises </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ProceedsFromOtherEquity_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Proceeds from the exercise of pre-funded warrants </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivities" xlink:label="us-gaap_NetCashProvidedByUsedInFinancingActivities" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivities" xlink:to="us-gaap_NetCashProvidedByUsedInFinancingActivities_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_NetCashProvidedByUsedInFinancingActivities_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Net Cash Provided by (Used in) Financing Activities </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_NetCashProvidedByUsedInFinancingActivities_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Net cash provided by financing activities </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents" xlink:label="us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents" xlink:to="us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Effect of Exchange Rate on Cash and Cash Equivalents </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Effect of foreign currency translation on cash </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect" xlink:label="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect" xlink:to="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Net increase (decrease) in cash and cash equivalents </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OrganizationConsolidationAndPresentationOfFinancialStatementsAbstract" xlink:label="us-gaap_OrganizationConsolidationAndPresentationOfFinancialStatementsAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OrganizationConsolidationAndPresentationOfFinancialStatementsAbstract" xlink:to="us-gaap_OrganizationConsolidationAndPresentationOfFinancialStatementsAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OrganizationConsolidationAndPresentationOfFinancialStatementsAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Nature of Business </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_NatureOfOperations" xlink:label="us-gaap_NatureOfOperations" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_NatureOfOperations" xlink:to="us-gaap_NatureOfOperations_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_NatureOfOperations_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Nature of Operations [Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_NatureOfOperations_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Nature of Business </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AccountingPoliciesAbstract" xlink:label="us-gaap_AccountingPoliciesAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_AccountingPoliciesAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AccountingPoliciesAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Summary of Significant Accounting Policies </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OrganizationConsolidationAndPresentationOfFinancialStatementsDisclosureAndSignificantAccountingPoliciesTextBlock" xlink:label="us-gaap_OrganizationConsolidationAndPresentationOfFinancialStatementsDisclosureAndSignificantAccountingPoliciesTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OrganizationConsolidationAndPresentationOfFinancialStatementsDisclosureAndSignificantAccountingPoliciesTextBlock" xlink:to="us-gaap_OrganizationConsolidationAndPresentationOfFinancialStatementsDisclosureAndSignificantAccountingPoliciesTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OrganizationConsolidationAndPresentationOfFinancialStatementsDisclosureAndSignificantAccountingPoliciesTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Organization, Consolidation and Presentation of Financial Statements Disclosure and Significant Accounting Policies [Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_OrganizationConsolidationAndPresentationOfFinancialStatementsDisclosureAndSignificantAccountingPoliciesTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Summary of Significant Accounting Policies </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RevenueFromContractWithCustomerAbstract" xlink:label="us-gaap_RevenueFromContractWithCustomerAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_RevenueFromContractWithCustomerAbstract" xlink:to="us-gaap_RevenueFromContractWithCustomerAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_RevenueFromContractWithCustomerAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Revenue. </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RevenueFromContractWithCustomerTextBlock" xlink:label="us-gaap_RevenueFromContractWithCustomerTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_RevenueFromContractWithCustomerTextBlock" xlink:to="us-gaap_RevenueFromContractWithCustomerTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_RevenueFromContractWithCustomerTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Revenue from Contract with Customer [Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_RevenueFromContractWithCustomerTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Revenue </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EarningsPerShareAbstract" xlink:label="us-gaap_EarningsPerShareAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_EarningsPerShareAbstract" xlink:to="us-gaap_EarningsPerShareAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_EarningsPerShareAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Net Loss Per Share of Common Stock </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EarningsPerShareTextBlock" xlink:label="us-gaap_EarningsPerShareTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_EarningsPerShareTextBlock" xlink:to="us-gaap_EarningsPerShareTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_EarningsPerShareTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Earnings Per Share [Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_EarningsPerShareTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Net Loss Per Share of Common Stock </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_WarrantsAndRightsNoteDisclosureAbstract" xlink:label="us-gaap_WarrantsAndRightsNoteDisclosureAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_WarrantsAndRightsNoteDisclosureAbstract" xlink:to="us-gaap_WarrantsAndRightsNoteDisclosureAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_WarrantsAndRightsNoteDisclosureAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Warrants </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_WarrantsTextBlock" xlink:label="ontx_WarrantsTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_WarrantsTextBlock" xlink:to="ontx_WarrantsTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_WarrantsTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> The entire disclosure for warrants. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_WarrantsTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Warrants [Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_WarrantsTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Warrants </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_BalanceSheetRelatedDisclosuresAbstract" xlink:label="us-gaap_BalanceSheetRelatedDisclosuresAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_BalanceSheetRelatedDisclosuresAbstract" xlink:to="us-gaap_BalanceSheetRelatedDisclosuresAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_BalanceSheetRelatedDisclosuresAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Balance Sheet Detail </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SupplementalBalanceSheetDisclosuresTextBlock" xlink:label="us-gaap_SupplementalBalanceSheetDisclosuresTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_SupplementalBalanceSheetDisclosuresTextBlock" xlink:to="us-gaap_SupplementalBalanceSheetDisclosuresTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_SupplementalBalanceSheetDisclosuresTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Supplemental Balance Sheet Disclosures [Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_SupplementalBalanceSheetDisclosuresTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Balance Sheet Detail </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueDisclosuresAbstract" xlink:label="us-gaap_FairValueDisclosuresAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueDisclosuresAbstract" xlink:to="us-gaap_FairValueDisclosuresAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueDisclosuresAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Fair Value Measurements </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueDisclosuresTextBlock" xlink:label="us-gaap_FairValueDisclosuresTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueDisclosuresTextBlock" xlink:to="us-gaap_FairValueDisclosuresTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueDisclosuresTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Fair Value Disclosures [Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_FairValueDisclosuresTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Fair Value Measurements </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_DisclosureOfCompensationRelatedCostsSharebasedPaymentsAbstract" xlink:label="us-gaap_DisclosureOfCompensationRelatedCostsSharebasedPaymentsAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_DisclosureOfCompensationRelatedCostsSharebasedPaymentsAbstract" xlink:to="us-gaap_DisclosureOfCompensationRelatedCostsSharebasedPaymentsAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_DisclosureOfCompensationRelatedCostsSharebasedPaymentsAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Stock-Based Compensation </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_DisclosureOfCompensationRelatedCostsShareBasedPaymentsTextBlock" xlink:label="us-gaap_DisclosureOfCompensationRelatedCostsShareBasedPaymentsTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_DisclosureOfCompensationRelatedCostsShareBasedPaymentsTextBlock" xlink:to="us-gaap_DisclosureOfCompensationRelatedCostsShareBasedPaymentsTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_DisclosureOfCompensationRelatedCostsShareBasedPaymentsTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Disclosure of Compensation Related Costs, Share-based Payments [Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_DisclosureOfCompensationRelatedCostsShareBasedPaymentsTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Stock-Based Compensation </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_ResearchAgreementsDisclosureAbstract" xlink:label="ontx_ResearchAgreementsDisclosureAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_ResearchAgreementsDisclosureAbstract" xlink:to="ontx_ResearchAgreementsDisclosureAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_ResearchAgreementsDisclosureAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Research Agreements </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_ResearchAgreementsDisclosureTextBlock" xlink:label="ontx_ResearchAgreementsDisclosureTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_ResearchAgreementsDisclosureTextBlock" xlink:to="ontx_ResearchAgreementsDisclosureTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_ResearchAgreementsDisclosureTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> The entire disclosure for research agreements. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_ResearchAgreementsDisclosureTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Research Agreements Disclosure [Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_ResearchAgreementsDisclosureTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Research Agreements </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_LicenseAndCollaborationAgreementsDisclosureAbstract" xlink:label="ontx_LicenseAndCollaborationAgreementsDisclosureAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_LicenseAndCollaborationAgreementsDisclosureAbstract" xlink:to="ontx_LicenseAndCollaborationAgreementsDisclosureAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_LicenseAndCollaborationAgreementsDisclosureAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> License and Collaboration Agreements </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CollaborativeArrangementDisclosureTextBlock" xlink:label="us-gaap_CollaborativeArrangementDisclosureTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_CollaborativeArrangementDisclosureTextBlock" xlink:to="us-gaap_CollaborativeArrangementDisclosureTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_CollaborativeArrangementDisclosureTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Collaborative Arrangement Disclosure [Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_CollaborativeArrangementDisclosureTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> License and Collaboration Agreements </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RelatedPartyTransactionsAbstract" xlink:label="us-gaap_RelatedPartyTransactionsAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_RelatedPartyTransactionsAbstract" xlink:to="us-gaap_RelatedPartyTransactionsAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_RelatedPartyTransactionsAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Related-Party Transactions </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RelatedPartyTransactionsDisclosureTextBlock" xlink:label="us-gaap_RelatedPartyTransactionsDisclosureTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_RelatedPartyTransactionsDisclosureTextBlock" xlink:to="us-gaap_RelatedPartyTransactionsDisclosureTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_RelatedPartyTransactionsDisclosureTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Related Party Transactions Disclosure [Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_RelatedPartyTransactionsDisclosureTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Related-Party Transactions </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_SecuritiesRegistrationAndSalesAgreementDisclosureAbstract" xlink:label="ontx_SecuritiesRegistrationAndSalesAgreementDisclosureAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_SecuritiesRegistrationAndSalesAgreementDisclosureAbstract" xlink:to="ontx_SecuritiesRegistrationAndSalesAgreementDisclosureAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_SecuritiesRegistrationAndSalesAgreementDisclosureAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> No definition available. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_SecuritiesRegistrationAndSalesAgreementDisclosureAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Securities Registrations and Sales Agreements </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_SecuritiesRegistrationAndSalesAgreementDisclosureTextBlock" xlink:label="ontx_SecuritiesRegistrationAndSalesAgreementDisclosureTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_SecuritiesRegistrationAndSalesAgreementDisclosureTextBlock" xlink:to="ontx_SecuritiesRegistrationAndSalesAgreementDisclosureTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_SecuritiesRegistrationAndSalesAgreementDisclosureTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> The entire disclosure for information about securities registration and sales agreement. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_SecuritiesRegistrationAndSalesAgreementDisclosureTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Securities Registration And Sales Agreement Disclosure [Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_SecuritiesRegistrationAndSalesAgreementDisclosureTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Securities Registrations and Sales Agreements </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_BasisOfAccountingPolicyPolicyTextBlock" xlink:label="us-gaap_BasisOfAccountingPolicyPolicyTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_BasisOfAccountingPolicyPolicyTextBlock" xlink:to="us-gaap_BasisOfAccountingPolicyPolicyTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_BasisOfAccountingPolicyPolicyTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Basis of Accounting, Policy [Policy Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_BasisOfAccountingPolicyPolicyTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Basis of Presentation </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_UnauditedInterimFinancialInformationPolicyTextBlock" xlink:label="ontx_UnauditedInterimFinancialInformationPolicyTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_UnauditedInterimFinancialInformationPolicyTextBlock" xlink:to="ontx_UnauditedInterimFinancialInformationPolicyTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_UnauditedInterimFinancialInformationPolicyTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Disclosure of accounting policy for unaudited interim financial information. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_UnauditedInterimFinancialInformationPolicyTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Unaudited Interim Financial Information Policy Text Block </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_UnauditedInterimFinancialInformationPolicyTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Unaudited Interim Financial Information </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SegmentReportingPolicyPolicyTextBlock" xlink:label="us-gaap_SegmentReportingPolicyPolicyTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_SegmentReportingPolicyPolicyTextBlock" xlink:to="us-gaap_SegmentReportingPolicyPolicyTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_SegmentReportingPolicyPolicyTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Segment Reporting, Policy [Policy Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_SegmentReportingPolicyPolicyTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Segment Information </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_SignificantAccountingPoliciesPolicyTextBlock" xlink:label="ontx_SignificantAccountingPoliciesPolicyTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_SignificantAccountingPoliciesPolicyTextBlock" xlink:to="ontx_SignificantAccountingPoliciesPolicyTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_SignificantAccountingPoliciesPolicyTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Disclosure of accounting policy for significant accounting policies. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_SignificantAccountingPoliciesPolicyTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Significant Accounting Policies Policy Text Block </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_SignificantAccountingPoliciesPolicyTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Significant Accounting Policies </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueMeasurementPolicyPolicyTextBlock" xlink:label="us-gaap_FairValueMeasurementPolicyPolicyTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueMeasurementPolicyPolicyTextBlock" xlink:to="us-gaap_FairValueMeasurementPolicyPolicyTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueMeasurementPolicyPolicyTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Fair Value Measurement, Policy [Policy Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_FairValueMeasurementPolicyPolicyTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Fair Value Measurements </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RevenueFromContractWithCustomerPolicyTextBlock" xlink:label="us-gaap_RevenueFromContractWithCustomerPolicyTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_RevenueFromContractWithCustomerPolicyTextBlock" xlink:to="us-gaap_RevenueFromContractWithCustomerPolicyTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_RevenueFromContractWithCustomerPolicyTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Revenue from Contract with Customer [Policy Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_RevenueFromContractWithCustomerPolicyTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Revenue Recognition </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_LesseeLeasesPolicyTextBlock" xlink:label="us-gaap_LesseeLeasesPolicyTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_LesseeLeasesPolicyTextBlock" xlink:to="us-gaap_LesseeLeasesPolicyTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_LesseeLeasesPolicyTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Lessee, Leases [Policy Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_LesseeLeasesPolicyTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Leases </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_NewAccountingPronouncementsPolicyPolicyTextBlock" xlink:label="us-gaap_NewAccountingPronouncementsPolicyPolicyTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_NewAccountingPronouncementsPolicyPolicyTextBlock" xlink:to="us-gaap_NewAccountingPronouncementsPolicyPolicyTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_NewAccountingPronouncementsPolicyPolicyTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> New Accounting Pronouncements, Policy [Policy Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_NewAccountingPronouncementsPolicyPolicyTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Recent Accounting Pronouncements </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTextBlock" xlink:label="us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTextBlock" xlink:to="us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Revenue [Table Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Schedule of recognized revenue under funding, license and collaboration agreements </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ContractWithCustomerAssetAndLiabilityTableTextBlock" xlink:label="us-gaap_ContractWithCustomerAssetAndLiabilityTableTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ContractWithCustomerAssetAndLiabilityTableTextBlock" xlink:to="us-gaap_ContractWithCustomerAssetAndLiabilityTableTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ContractWithCustomerAssetAndLiabilityTableTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Contract with Customer, Asset and Liability [Table Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ContractWithCustomerAssetAndLiabilityTableTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Schedule of deferred revenue </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTextBlock" xlink:label="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTextBlock" xlink:to="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Schedule of antidilutive securities which have been excluded from the computation of diluted weighted average shares outstanding </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ScheduleOfStockholdersEquityNoteWarrantsOrRightsTextBlock" xlink:label="us-gaap_ScheduleOfStockholdersEquityNoteWarrantsOrRightsTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ScheduleOfStockholdersEquityNoteWarrantsOrRightsTextBlock" xlink:to="us-gaap_ScheduleOfStockholdersEquityNoteWarrantsOrRightsTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ScheduleOfStockholdersEquityNoteWarrantsOrRightsTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Schedule of Stockholders' Equity Note, Warrants or Rights [Table Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ScheduleOfStockholdersEquityNoteWarrantsOrRightsTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Schedule of warrants outstanding and warrant activity </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ScheduleOfOtherCurrentAssetsTableTextBlock" xlink:label="us-gaap_ScheduleOfOtherCurrentAssetsTableTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ScheduleOfOtherCurrentAssetsTableTextBlock" xlink:to="us-gaap_ScheduleOfOtherCurrentAssetsTableTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ScheduleOfOtherCurrentAssetsTableTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Schedule of Prepaid Expenses and Other Current Assets [Table Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ScheduleOfOtherCurrentAssetsTableTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Schedule of prepaid expenses and other current assets </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTextBlock" xlink:label="us-gaap_PropertyPlantAndEquipmentTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_PropertyPlantAndEquipmentTextBlock" xlink:to="us-gaap_PropertyPlantAndEquipmentTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_PropertyPlantAndEquipmentTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Property, Plant and Equipment [Table Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_PropertyPlantAndEquipmentTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Schedule of property and equipment </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ScheduleOfAccruedLiabilitiesTableTextBlock" xlink:label="us-gaap_ScheduleOfAccruedLiabilitiesTableTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ScheduleOfAccruedLiabilitiesTableTextBlock" xlink:to="us-gaap_ScheduleOfAccruedLiabilitiesTableTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ScheduleOfAccruedLiabilitiesTableTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Schedule of Accrued Expenses and Other Current Liabilities [Table Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ScheduleOfAccruedLiabilitiesTableTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Schedule of accrued expenses and other current liabilities </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTableTextBlock" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTableTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTableTextBlock" xlink:to="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTableTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTableTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Schedule of Black-Scholes option pricing model assumptions </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ScheduleOfFairValueAssetsAndLiabilitiesMeasuredOnRecurringBasisTableTextBlock" xlink:label="us-gaap_ScheduleOfFairValueAssetsAndLiabilitiesMeasuredOnRecurringBasisTableTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ScheduleOfFairValueAssetsAndLiabilitiesMeasuredOnRecurringBasisTableTextBlock" xlink:to="us-gaap_ScheduleOfFairValueAssetsAndLiabilitiesMeasuredOnRecurringBasisTableTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ScheduleOfFairValueAssetsAndLiabilitiesMeasuredOnRecurringBasisTableTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ScheduleOfFairValueAssetsAndLiabilitiesMeasuredOnRecurringBasisTableTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Schedule of financial assets and liabilities measured at fair value on a recurring basis </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTextBlock" xlink:label="us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTextBlock" xlink:to="us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Schedule of stock-based compensation expense </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationStockOptionsActivityTableTextBlock" xlink:label="us-gaap_ScheduleOfShareBasedCompensationStockOptionsActivityTableTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ScheduleOfShareBasedCompensationStockOptionsActivityTableTextBlock" xlink:to="us-gaap_ScheduleOfShareBasedCompensationStockOptionsActivityTableTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ScheduleOfShareBasedCompensationStockOptionsActivityTableTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Schedule of Share-based Compensation, Stock Options, Activity [Table Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ScheduleOfShareBasedCompensationStockOptionsActivityTableTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Schedule of stock option activity </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeTextBlock" xlink:label="us-gaap_ScheduleOfShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ScheduleOfShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeTextBlock" xlink:to="us-gaap_ScheduleOfShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ScheduleOfShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Schedule of Share-based Compensation, Shares Authorized under Stock Option Plans, by Exercise Price Range [Table Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ScheduleOfShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Schedule of information with respect to stock options outstanding and exercisable </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ScheduleOfShareBasedPaymentAwardStockOptionsValuationAssumptionsTableTextBlock" xlink:label="us-gaap_ScheduleOfShareBasedPaymentAwardStockOptionsValuationAssumptionsTableTextBlock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ScheduleOfShareBasedPaymentAwardStockOptionsValuationAssumptionsTableTextBlock" xlink:to="us-gaap_ScheduleOfShareBasedPaymentAwardStockOptionsValuationAssumptionsTableTextBlock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ScheduleOfShareBasedPaymentAwardStockOptionsValuationAssumptionsTableTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ScheduleOfShareBasedPaymentAwardStockOptionsValuationAssumptionsTableTextBlock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Schedule of weighted-average assumptions used for estimating the fair value of the stock compensation granted </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SubsidiaryOrEquityMethodInvesteeSaleOfStockBySubsidiaryOrEquityInvesteeTable" xlink:label="us-gaap_SubsidiaryOrEquityMethodInvesteeSaleOfStockBySubsidiaryOrEquityInvesteeTable" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_SubsidiaryOrEquityMethodInvesteeSaleOfStockBySubsidiaryOrEquityInvesteeTable" xlink:to="us-gaap_SubsidiaryOrEquityMethodInvesteeSaleOfStockBySubsidiaryOrEquityInvesteeTable_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_SubsidiaryOrEquityMethodInvesteeSaleOfStockBySubsidiaryOrEquityInvesteeTable_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Subsidiary or Equity Method Investee, Sale of Stock by Subsidiary or Equity Investee [Table] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SubsidiarySaleOfStockLineItems" xlink:label="us-gaap_SubsidiarySaleOfStockLineItems" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_SubsidiarySaleOfStockLineItems" xlink:to="us-gaap_SubsidiarySaleOfStockLineItems_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_SubsidiarySaleOfStockLineItems_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Subsidiary, Sale of Stock [Line Items] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_SubsidiarySaleOfStockLineItems_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Liquidity </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights" xlink:label="us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights" xlink:to="us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Class of Warrant or Right, Number of Securities Called by Warrants or Rights </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Stock called by warrants or rights </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SharePrice" xlink:label="us-gaap_SharePrice" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_SharePrice" xlink:to="us-gaap_SharePrice_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_SharePrice_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share Price </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_SharePrice_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Price per share (in dollars per share) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RestructuringAndRelatedCostNumberOfPositionsEliminated" xlink:label="us-gaap_RestructuringAndRelatedCostNumberOfPositionsEliminated" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_RestructuringAndRelatedCostNumberOfPositionsEliminated" xlink:to="us-gaap_RestructuringAndRelatedCostNumberOfPositionsEliminated_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_RestructuringAndRelatedCostNumberOfPositionsEliminated_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Restructuring and Related Cost, Number of Positions Eliminated </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_RestructuringAndRelatedCostNumberOfPositionsEliminated_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> FTE positions eliminated </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RestructuringAndRelatedCostNumberOfPositionsEliminatedPeriodPercent" xlink:label="us-gaap_RestructuringAndRelatedCostNumberOfPositionsEliminatedPeriodPercent" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_RestructuringAndRelatedCostNumberOfPositionsEliminatedPeriodPercent" xlink:to="us-gaap_RestructuringAndRelatedCostNumberOfPositionsEliminatedPeriodPercent_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_RestructuringAndRelatedCostNumberOfPositionsEliminatedPeriodPercent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Restructuring and Related Cost, Number of Positions Eliminated, Period Percent </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_RestructuringAndRelatedCostNumberOfPositionsEliminatedPeriodPercent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> FTE eliminated positions (as percent of workforce) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SeveranceCosts1" xlink:label="us-gaap_SeveranceCosts1" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_SeveranceCosts1" xlink:to="us-gaap_SeveranceCosts1_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_SeveranceCosts1_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Severance Costs </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_SeveranceCosts1_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Severance charge </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AcceleratedShareRepurchasesAdjustmentToRecordedAmount" xlink:label="us-gaap_AcceleratedShareRepurchasesAdjustmentToRecordedAmount" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_AcceleratedShareRepurchasesAdjustmentToRecordedAmount" xlink:to="us-gaap_AcceleratedShareRepurchasesAdjustmentToRecordedAmount_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AcceleratedShareRepurchasesAdjustmentToRecordedAmount_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Accelerated Share Repurchases, Adjustment to Recorded Amount </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_AcceleratedShareRepurchasesAdjustmentToRecordedAmount_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non-cash charge, accelerated options </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_NumberOfOperatingSegments" xlink:label="us-gaap_NumberOfOperatingSegments" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_NumberOfOperatingSegments" xlink:to="us-gaap_NumberOfOperatingSegments_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_NumberOfOperatingSegments_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Number of Operating Segments </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:label="us-gaap_NumberOfOperatingSegments_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Number of operating segments </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_NumberOfLeases" xlink:label="ontx_NumberOfLeases" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_NumberOfLeases" xlink:to="ontx_NumberOfLeases_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_NumberOfLeases_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Represents the number of leases. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_NumberOfLeases_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Number of leases </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_NumberOfLeases_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Number of leases </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OperatingLeasePayments" xlink:label="us-gaap_OperatingLeasePayments" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OperatingLeasePayments" xlink:to="us-gaap_OperatingLeasePayments_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OperatingLeasePayments_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Operating Lease, Payments </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_OperatingLeasePayments_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Amount of lease payments </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_OperatingLeasePaymentsDue" xlink:label="ontx_OperatingLeasePaymentsDue" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_OperatingLeasePaymentsDue" xlink:to="ontx_OperatingLeasePaymentsDue_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_OperatingLeasePaymentsDue_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Amount of lease payments due. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_OperatingLeasePaymentsDue_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Operating lease payments due </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_OperatingLeasePaymentsDue_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Remaining payments due </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OperatingLeaseLiability" xlink:label="us-gaap_OperatingLeaseLiability" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OperatingLeaseLiability" xlink:to="us-gaap_OperatingLeaseLiability_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OperatingLeaseLiability_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Operating Lease, Liability </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_OperatingLeaseLiability_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Amount of lease liabilities </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OperatingLeaseRightOfUseAsset" xlink:label="us-gaap_OperatingLeaseRightOfUseAsset" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OperatingLeaseRightOfUseAsset" xlink:to="us-gaap_OperatingLeaseRightOfUseAsset_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OperatingLeaseRightOfUseAsset_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Operating Lease, Right-of-Use Asset </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_OperatingLeaseRightOfUseAsset_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Amount of right of use asset </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" xlink:label="us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" xlink:to="us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Schedule of Collaborative Arrangements and Non-collaborative Arrangement Transactions [Table] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:label="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> License and collaboration agreements </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:label="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Research Agreements </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RevenuesAbstract" xlink:label="us-gaap_RevenuesAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_RevenuesAbstract" xlink:to="us-gaap_RevenuesAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_RevenuesAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Revenues [Abstract] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_RevenuesAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Revenue </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_MovementInDeferredRevenueRollForward" xlink:label="us-gaap_MovementInDeferredRevenueRollForward" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_MovementInDeferredRevenueRollForward" xlink:to="us-gaap_MovementInDeferredRevenueRollForward_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_MovementInDeferredRevenueRollForward_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Movement in Deferred Revenue [Roll Forward] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_MovementInDeferredRevenueRollForward_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Deferred revenue </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ContractWithCustomerLiabilityRevenueRecognized" xlink:label="us-gaap_ContractWithCustomerLiabilityRevenueRecognized" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ContractWithCustomerLiabilityRevenueRecognized" xlink:to="us-gaap_ContractWithCustomerLiabilityRevenueRecognized_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ContractWithCustomerLiabilityRevenueRecognized_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Contract with Customer, Liability, Revenue Recognized </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:label="us-gaap_ContractWithCustomerLiabilityRevenueRecognized_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Recognition to revenue </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable" xlink:label="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable" xlink:to="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" xlink:to="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Antidilutive Securities [Axis] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AntidilutiveSecuritiesNameDomain" xlink:label="us-gaap_AntidilutiveSecuritiesNameDomain" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_AntidilutiveSecuritiesNameDomain" xlink:to="us-gaap_AntidilutiveSecuritiesNameDomain_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AntidilutiveSecuritiesNameDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Antidilutive Securities Name [Domain] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_AntidilutiveSecuritiesNameDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Antidilutive Securities, Name [Domain] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_WarrantMember" xlink:label="us-gaap_WarrantMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_WarrantMember" xlink:to="us-gaap_WarrantMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_WarrantMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Warrant [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_WarrantMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Warrants </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="us-gaap_EmployeeStockOptionMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_EmployeeStockOptionMember" xlink:to="us-gaap_EmployeeStockOptionMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_EmployeeStockOptionMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Employee Stock Option [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_EmployeeStockOptionMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Stock options </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems" xlink:to="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Potentially dilutive securities outstanding excluded from the computation of diluted weighted average shares as they would be antidilutive </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" xlink:to="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Potentially dilutive securities outstanding excluded from the computation of diluted weighted average shares </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ClassOfWarrantOrRightTable" xlink:label="us-gaap_ClassOfWarrantOrRightTable" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ClassOfWarrantOrRightTable" xlink:to="us-gaap_ClassOfWarrantOrRightTable_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ClassOfWarrantOrRightTable_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Class of Warrant or Right [Table] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ClassOfWarrantOrRightLineItems" xlink:label="us-gaap_ClassOfWarrantOrRightLineItems" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ClassOfWarrantOrRightLineItems" xlink:to="us-gaap_ClassOfWarrantOrRightLineItems_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ClassOfWarrantOrRightLineItems_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Class of Warrant or Right [Line Items] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ClassOfWarrantOrRightLineItems_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Warrants </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1" xlink:label="us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1" xlink:to="us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Class of Warrant or Right, Exercise Price of Warrants or Rights </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Exercise price (in dollars per share) </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:label="us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Warrant exercise price (in dollars per share) </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_ClassOfWarrantOrRightRollForward" xlink:label="ontx_ClassOfWarrantOrRightRollForward" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_ClassOfWarrantOrRightRollForward" xlink:to="ontx_ClassOfWarrantOrRightRollForward_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_ClassOfWarrantOrRightRollForward_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Class of Warrant or Right [Roll Forward] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_ClassOfWarrantOrRightRollForward_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Class of Warrant or Right [Roll Forward] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_ClassOfWarrantOrRightRollForward_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Warrants outstanding and warrant activity </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ClassOfWarrantOrRightOutstanding" xlink:label="us-gaap_ClassOfWarrantOrRightOutstanding" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ClassOfWarrantOrRightOutstanding" xlink:to="us-gaap_ClassOfWarrantOrRightOutstanding_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ClassOfWarrantOrRightOutstanding_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Class of Warrant or Right, Outstanding </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel" xlink:label="us-gaap_ClassOfWarrantOrRightOutstanding_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Balance at end of the period (in shares) </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/periodStartLabel" xlink:label="us-gaap_ClassOfWarrantOrRightOutstanding_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Balance at beginning of the period (in shares) </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_ClassOfWarrantOrRightIssued" xlink:label="ontx_ClassOfWarrantOrRightIssued" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_ClassOfWarrantOrRightIssued" xlink:to="ontx_ClassOfWarrantOrRightIssued_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_ClassOfWarrantOrRightIssued_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> The number of warrants or rights issued during the period. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_ClassOfWarrantOrRightIssued_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Class of Warrant or Right, Issued </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_ClassOfWarrantOrRightIssued_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Warrants Issued (in shares) </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_ClassOfWarrantOrRightExercised" xlink:label="ontx_ClassOfWarrantOrRightExercised" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_ClassOfWarrantOrRightExercised" xlink:to="ontx_ClassOfWarrantOrRightExercised_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_ClassOfWarrantOrRightExercised_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Number of warrants exercised during the current period. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_ClassOfWarrantOrRightExercised_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Class of Warrant or Right, Exercised </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2009/role/negatedLabel" xlink:label="ontx_ClassOfWarrantOrRightExercised_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Warrants Exercised (in shares) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PrepaidExpenseAndOtherAssetsCurrentAbstract" xlink:label="us-gaap_PrepaidExpenseAndOtherAssetsCurrentAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_PrepaidExpenseAndOtherAssetsCurrentAbstract" xlink:to="us-gaap_PrepaidExpenseAndOtherAssetsCurrentAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_PrepaidExpenseAndOtherAssetsCurrentAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Prepaid Expense and Other Assets, Current [Abstract] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_PrepaidExpenseAndOtherAssetsCurrentAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Prepaid expenses and other current assets: </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_PrepaidResearchAndDevelopmentCurrent" xlink:label="ontx_PrepaidResearchAndDevelopmentCurrent" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_PrepaidResearchAndDevelopmentCurrent" xlink:to="ontx_PrepaidResearchAndDevelopmentCurrent_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_PrepaidResearchAndDevelopmentCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Amount of asset related to consideration paid in advance for research and development that provides economic benefits within a future period of one year or the normal operating cycle, if longer. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_PrepaidResearchAndDevelopmentCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Prepaid Research and Development, Current </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_PrepaidResearchAndDevelopmentCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Research and development </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OtherPrepaidExpenseCurrent" xlink:label="us-gaap_OtherPrepaidExpenseCurrent" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OtherPrepaidExpenseCurrent" xlink:to="us-gaap_OtherPrepaidExpenseCurrent_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OtherPrepaidExpenseCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Prepaid Manufacturing </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_OtherPrepaidExpenseCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Manufacturing </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PrepaidInsurance" xlink:label="us-gaap_PrepaidInsurance" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_PrepaidInsurance" xlink:to="us-gaap_PrepaidInsurance_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_PrepaidInsurance_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Prepaid Insurance </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_PrepaidInsurance_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Insurance </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OtherAssetsCurrent" xlink:label="us-gaap_OtherAssetsCurrent" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OtherAssetsCurrent" xlink:to="us-gaap_OtherAssetsCurrent_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OtherAssetsCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Other Assets, Current </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_OtherAssetsCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Other </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PropertyPlantAndEquipmentAbstract" xlink:label="us-gaap_PropertyPlantAndEquipmentAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_PropertyPlantAndEquipmentAbstract" xlink:to="us-gaap_PropertyPlantAndEquipmentAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_PropertyPlantAndEquipmentAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Property and Equipment </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PropertyPlantAndEquipmentGross" xlink:label="us-gaap_PropertyPlantAndEquipmentGross" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_PropertyPlantAndEquipmentGross" xlink:to="us-gaap_PropertyPlantAndEquipmentGross_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_PropertyPlantAndEquipmentGross_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Property, Plant and Equipment, Gross </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_PropertyPlantAndEquipmentGross_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Property and equipment </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:label="us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:to="us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2009/role/negatedLabel" xlink:label="us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Accumulated depreciation </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_AccruedResearchAndDevelopmentCurrent" xlink:label="ontx_AccruedResearchAndDevelopmentCurrent" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_AccruedResearchAndDevelopmentCurrent" xlink:to="ontx_AccruedResearchAndDevelopmentCurrent_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_AccruedResearchAndDevelopmentCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Carrying value as of the balance sheet date of obligations incurred through that date and payable for research and development. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer). </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_AccruedResearchAndDevelopmentCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Accrued Research and Development, Current </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_AccruedResearchAndDevelopmentCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Research and development </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EmployeeRelatedLiabilitiesCurrent" xlink:label="us-gaap_EmployeeRelatedLiabilitiesCurrent" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_EmployeeRelatedLiabilitiesCurrent" xlink:to="us-gaap_EmployeeRelatedLiabilitiesCurrent_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_EmployeeRelatedLiabilitiesCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Employee-related Liabilities, Current </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_EmployeeRelatedLiabilitiesCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Employee compensation </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AccruedProfessionalFeesCurrent" xlink:label="us-gaap_AccruedProfessionalFeesCurrent" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_AccruedProfessionalFeesCurrent" xlink:to="us-gaap_AccruedProfessionalFeesCurrent_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AccruedProfessionalFeesCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Accrued Professional Fees, Current </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_AccruedProfessionalFeesCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Professional fees </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ProceedsFromIssuanceOrSaleOfEquityAbstract" xlink:label="us-gaap_ProceedsFromIssuanceOrSaleOfEquityAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ProceedsFromIssuanceOrSaleOfEquityAbstract" xlink:to="us-gaap_ProceedsFromIssuanceOrSaleOfEquityAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ProceedsFromIssuanceOrSaleOfEquityAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Proceeds from Issuance or Sale of Equity [Abstract] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:label="us-gaap_ProceedsFromIssuanceOrSaleOfEquityAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Sale of Securities </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ProceedsFromIssuanceOrSaleOfEquity" xlink:label="us-gaap_ProceedsFromIssuanceOrSaleOfEquity" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ProceedsFromIssuanceOrSaleOfEquity" xlink:to="us-gaap_ProceedsFromIssuanceOrSaleOfEquity_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ProceedsFromIssuanceOrSaleOfEquity_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> ProceedsFromIssuanceOrSa </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ProceedsFromIssuanceOrSaleOfEquity_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> ProceedsFromIssuanceOrSaleOfEquity </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueHedgesAtFairValueNet" xlink:label="us-gaap_FairValueHedgesAtFairValueNet" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueHedgesAtFairValueNet" xlink:to="us-gaap_FairValueHedgesAtFairValueNet_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueHedgesAtFairValueNet_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Fair Value Hedges, Net </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:label="us-gaap_FairValueHedgesAtFairValueNet_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Estimated fair value using the Black-Scholes pricing model </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_ClassOfWarrantOrRightNumberOfWarrantsOrRightsIssued" xlink:label="ontx_ClassOfWarrantOrRightNumberOfWarrantsOrRightsIssued" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_ClassOfWarrantOrRightNumberOfWarrantsOrRightsIssued" xlink:to="ontx_ClassOfWarrantOrRightNumberOfWarrantsOrRightsIssued_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_ClassOfWarrantOrRightNumberOfWarrantsOrRightsIssued_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Number of warrants or rights issued during the period. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_ClassOfWarrantOrRightNumberOfWarrantsOrRightsIssued_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Class of Warrant or Right, Number of Warrants or Rights Issued </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:label="ontx_ClassOfWarrantOrRightNumberOfWarrantsOrRightsIssued_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Rights or warrants issued (in shares) </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_ClassOfWarrantOrRightInitiallyExercisableTerm" xlink:label="ontx_ClassOfWarrantOrRightInitiallyExercisableTerm" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_ClassOfWarrantOrRightInitiallyExercisableTerm" xlink:to="ontx_ClassOfWarrantOrRightInitiallyExercisableTerm_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_ClassOfWarrantOrRightInitiallyExercisableTerm_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Term of initially exercisable warrants or rights outstanding. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_ClassOfWarrantOrRightInitiallyExercisableTerm_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Class Of Warrant Or Right Initially Exercisable Term </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_ClassOfWarrantOrRightRedemptionPeriod" xlink:label="ontx_ClassOfWarrantOrRightRedemptionPeriod" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_ClassOfWarrantOrRightRedemptionPeriod" xlink:to="ontx_ClassOfWarrantOrRightRedemptionPeriod_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_ClassOfWarrantOrRightRedemptionPeriod_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> The period after which the warrants or rights can redeemed at a stated redemption price. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_ClassOfWarrantOrRightRedemptionPeriod_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Class of Warrant or Right, Redemption Period </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_ClassOfWarrantOrRightRedemptionPeriod_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Redemption period (in years) </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_ClassOfWarrantOrRightRedemptionPrice" xlink:label="ontx_ClassOfWarrantOrRightRedemptionPrice" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_ClassOfWarrantOrRightRedemptionPrice" xlink:to="ontx_ClassOfWarrantOrRightRedemptionPrice_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_ClassOfWarrantOrRightRedemptionPrice_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> The redemption price of warrant or right at which it is redeemed after the redemption period. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_ClassOfWarrantOrRightRedemptionPrice_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Class of Warrant or Right, Redemption Price </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_ClassOfWarrantOrRightRedemptionPrice_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Redemption price (in dollars per share) </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_ClassOfWarrantOrRightRedeemableVolumeWeightedAverageCommonStockPriceThreshold" xlink:label="ontx_ClassOfWarrantOrRightRedeemableVolumeWeightedAverageCommonStockPriceThreshold" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_ClassOfWarrantOrRightRedeemableVolumeWeightedAverageCommonStockPriceThreshold" xlink:to="ontx_ClassOfWarrantOrRightRedeemableVolumeWeightedAverageCommonStockPriceThreshold_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_ClassOfWarrantOrRightRedeemableVolumeWeightedAverageCommonStockPriceThreshold_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Common stock volume weighted average price threshold for redeeming a class of right or warrant. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_ClassOfWarrantOrRightRedeemableVolumeWeightedAverageCommonStockPriceThreshold_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Class of Warrant or Right, Redeemable, Volume Weighted Average Common Stock Price Threshold </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_ClassOfWarrantOrRightRedeemableVolumeWeightedAverageCommonStockPriceThreshold_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Volume weighted average price per share of common stock (in dollars per share) </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_ClassOfWarrantOrRightRedemptionThresholdConsecutiveTradingDays" xlink:label="ontx_ClassOfWarrantOrRightRedemptionThresholdConsecutiveTradingDays" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_ClassOfWarrantOrRightRedemptionThresholdConsecutiveTradingDays" xlink:to="ontx_ClassOfWarrantOrRightRedemptionThresholdConsecutiveTradingDays_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_ClassOfWarrantOrRightRedemptionThresholdConsecutiveTradingDays_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Threshold period of specified consecutive trading days during which the volume weighted average price of stock is above a particular price. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_ClassOfWarrantOrRightRedemptionThresholdConsecutiveTradingDays_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Class of Warrant or Right, Redemption, Threshold, Consecutive Trading Days </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_ClassOfWarrantOrRightRedemptionThresholdConsecutiveTradingDays_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Threshold consecutive trading days (in days) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable" xlink:to="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Fair Value Measurement Inputs and Valuation Techniques [Table] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems" xlink:to="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Fair Value Measurement Inputs and Valuation Techniques [Line Items] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_DerivativeLiabilityMeasurementInput" xlink:label="us-gaap_DerivativeLiabilityMeasurementInput" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_DerivativeLiabilityMeasurementInput" xlink:to="us-gaap_DerivativeLiabilityMeasurementInput_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_DerivativeLiabilityMeasurementInput_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Derivative Liability, Measurement Input </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_DerivativeLiabilityMeasurementInput_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Measurement input </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:to="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Fair Value Measurements, Recurring and Nonrecurring [Table] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:to="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_LiabilitiesFairValueDisclosureAbstract" xlink:label="us-gaap_LiabilitiesFairValueDisclosureAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_LiabilitiesFairValueDisclosureAbstract" xlink:to="us-gaap_LiabilitiesFairValueDisclosureAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_LiabilitiesFairValueDisclosureAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Liabilities, Fair Value Disclosure [Abstract] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_LiabilitiesFairValueDisclosureAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Liabilities measured at fair value </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FinancialLiabilitiesFairValueDisclosure" xlink:label="us-gaap_FinancialLiabilitiesFairValueDisclosure" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FinancialLiabilitiesFairValueDisclosure" xlink:to="us-gaap_FinancialLiabilitiesFairValueDisclosure_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FinancialLiabilitiesFairValueDisclosure_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Financial Liabilities Fair Value Disclosure </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_FinancialLiabilitiesFairValueDisclosure_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Total </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueTransfersBetweenLevel1AndLevel2DescriptionAndPolicyAbstract" xlink:label="us-gaap_FairValueTransfersBetweenLevel1AndLevel2DescriptionAndPolicyAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueTransfersBetweenLevel1AndLevel2DescriptionAndPolicyAbstract" xlink:to="us-gaap_FairValueTransfersBetweenLevel1AndLevel2DescriptionAndPolicyAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueTransfersBetweenLevel1AndLevel2DescriptionAndPolicyAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Fair Value, Transfers Between Level 1 and Level 2, Description and Policy [Abstract] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_FairValueTransfersBetweenLevel1AndLevel2DescriptionAndPolicyAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Fair Value Measurements </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount" xlink:label="us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount" xlink:to="us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Fair Value, Assets, Level 1 to Level 2 Transfers, Amount </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Amount of transfers of assets out of Level 1 into Level 2 </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount" xlink:label="us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount" xlink:to="us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Fair Value, Assets, Level 2 to Level 1 Transfers, Amount </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Amount of transfers of assets out of Level 2 into Level 1 </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueLiabilitiesLevel1ToLevel2TransfersAmount" xlink:label="us-gaap_FairValueLiabilitiesLevel1ToLevel2TransfersAmount" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueLiabilitiesLevel1ToLevel2TransfersAmount" xlink:to="us-gaap_FairValueLiabilitiesLevel1ToLevel2TransfersAmount_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueLiabilitiesLevel1ToLevel2TransfersAmount_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_FairValueLiabilitiesLevel1ToLevel2TransfersAmount_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Amount of transfers of liabilities out of Level 1 into Level 2 </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueLiabilitiesLevel2ToLevel1TransfersAmount" xlink:label="us-gaap_FairValueLiabilitiesLevel2ToLevel1TransfersAmount" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueLiabilitiesLevel2ToLevel1TransfersAmount" xlink:to="us-gaap_FairValueLiabilitiesLevel2ToLevel1TransfersAmount_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueLiabilitiesLevel2ToLevel1TransfersAmount_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Fair Value, Liabilities, Level 2 to Level 1 Transfers, Amount </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_FairValueLiabilitiesLevel2ToLevel1TransfersAmount_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Amount of transfers of liabilities out of Level 2 into Level 1 </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award [Line Items] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAdditionalGeneralDisclosuresAbstract" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAdditionalGeneralDisclosuresAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAdditionalGeneralDisclosuresAbstract" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAdditionalGeneralDisclosuresAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAdditionalGeneralDisclosuresAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Additional General Disclosures [Abstract] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAdditionalGeneralDisclosuresAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Stock-Based Compensation </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Shares authorized (in shares) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Additional shares authorized (in shares) </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_CommonStockCapitalSharesReservedForFutureIssuanceEvergreenProvisionSharesAsPercentageOfIssuedAndOutstandingCommonStock" xlink:label="ontx_CommonStockCapitalSharesReservedForFutureIssuanceEvergreenProvisionSharesAsPercentageOfIssuedAndOutstandingCommonStock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_CommonStockCapitalSharesReservedForFutureIssuanceEvergreenProvisionSharesAsPercentageOfIssuedAndOutstandingCommonStock" xlink:to="ontx_CommonStockCapitalSharesReservedForFutureIssuanceEvergreenProvisionSharesAsPercentageOfIssuedAndOutstandingCommonStock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_CommonStockCapitalSharesReservedForFutureIssuanceEvergreenProvisionSharesAsPercentageOfIssuedAndOutstandingCommonStock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Evergreen provision, percentage of issued and outstanding common stock. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_CommonStockCapitalSharesReservedForFutureIssuanceEvergreenProvisionSharesAsPercentageOfIssuedAndOutstandingCommonStock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Common Stock, Capital Shares Reserved for Future Issuance, Evergreen Provision, Shares as Percentage of Issued and Outstanding Common Stock </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_CommonStockCapitalSharesReservedForFutureIssuanceEvergreenProvisionSharesAsPercentageOfIssuedAndOutstandingCommonStock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Evergreen provision (percent) </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_CommonStockCapitalSharesReservedForFutureIssuanceEvergreenProvisionShares" xlink:label="ontx_CommonStockCapitalSharesReservedForFutureIssuanceEvergreenProvisionShares" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_CommonStockCapitalSharesReservedForFutureIssuanceEvergreenProvisionShares" xlink:to="ontx_CommonStockCapitalSharesReservedForFutureIssuanceEvergreenProvisionShares_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_CommonStockCapitalSharesReservedForFutureIssuanceEvergreenProvisionShares_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Evergreen provision, shares </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_CommonStockCapitalSharesReservedForFutureIssuanceEvergreenProvisionShares_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Common Stock, Capital Shares Reserved for Future Issuance, Evergreen Provision, Shares </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_CommonStockCapitalSharesReservedForFutureIssuanceEvergreenProvisionShares_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Evergreen provision, shares ( in shares) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Balance at the end of the period (in shares) </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/periodStartLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Balance at the beginning of the period (in shares) </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Common Stock available for future issuance (in shares) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable" xlink:label="us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable" xlink:to="us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:to="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationAggregateDisclosuresAbstract" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationAggregateDisclosuresAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAggregateDisclosuresAbstract" xlink:to="us-gaap_EmployeeServiceShareBasedCompensationAggregateDisclosuresAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationAggregateDisclosuresAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Employee Service Share-based Compensation, Aggregate Disclosures [Abstract] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationAggregateDisclosuresAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Stock-Based Compensation </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense" xlink:to="us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Employee Service Share-based Compensation, Tax Benefit from Compensation Expense </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Net tax benefits related to the stock-based compensation costs </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="us-gaap_AllocatedShareBasedCompensationExpense" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_AllocatedShareBasedCompensationExpense" xlink:to="us-gaap_AllocatedShareBasedCompensationExpense_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AllocatedShareBasedCompensationExpense_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Allocated Share-based Compensation Expense </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:label="us-gaap_AllocatedShareBasedCompensationExpense_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Total stock-based compensation </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantRollForward" xlink:label="ontx_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantRollForward" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantRollForward" xlink:to="ontx_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantRollForward_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantRollForward_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share Based Compensation Arrangement by Share Based Payment Award, Number of Shares Available for Grant [Roll Forward] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantRollForward_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share Based Compensation Arrangement by Share Based Payment Award, Number of Shares Available for Grant [Roll Forward] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:label="ontx_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantRollForward_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Shares Available for Grant </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Granted (in shares) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Options, Expirations in Period </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2009/role/negatedLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Cancelled (in shares) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Number of Shares </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Balance at the end of the period (in shares) </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/periodStartLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Balance at the beginning of the period (in shares) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures in Period </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2009/role/negatedLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Forfeitures (in shares) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Number </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Vested or expected to vest at the end of the period (in shares) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Exercisable at the end of the period (in shares) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Weighted-Average Exercise Price </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Balance at the end of the period (in dollars per share) </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/periodStartLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Balance at the beginning of the period (in dollars per share) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Granted (in dollars per share) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangements by Share-based Payment Award, Options, Forfeitures in Period, Weighted Average Exercise Price </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Forfeitures (in dollars per share) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Weighted Average Exercise Price </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Vested or expected to vest at the end of the period (in dollars per share) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Exercisable at the end of the period (in dollars per share) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Additional Disclosures </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Stock-Based Compensation </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Weighted average remaining contractual term </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresWeightedAverageRemainingContractualTerm" xlink:label="ontx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresWeightedAverageRemainingContractualTerm" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresWeightedAverageRemainingContractualTerm" xlink:to="ontx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresWeightedAverageRemainingContractualTerm_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresWeightedAverageRemainingContractualTerm_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Weighted average remaining contractual term for vested portions of options outstanding and currently forfeitures or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresWeightedAverageRemainingContractualTerm_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures, Weighted Average Remaining Contractual Term </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresWeightedAverageRemainingContractualTerm_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Weighted average remaining contractual term of options forfeitures </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantedWeightedAverageRemainingContractualTerm" xlink:label="ontx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantedWeightedAverageRemainingContractualTerm" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantedWeightedAverageRemainingContractualTerm" xlink:to="ontx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantedWeightedAverageRemainingContractualTerm_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantedWeightedAverageRemainingContractualTerm_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Weighted average remaining contractual term for vested portions of options outstanding and currently or granted convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantedWeightedAverageRemainingContractualTerm_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Options, Granted, Weighted Average Remaining Contractual Term </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantedWeightedAverageRemainingContractualTerm_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Weighted average remaining contractual term of options granted </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Weighted Average Remaining Contractual Term </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Weighted average remaining contractual term of options vested or expected to vest </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Weighted average remaining contractual term of options exercisable </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Aggregate intrinsic value of options outstanding </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Aggregate Intrinsic Value </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Aggregate intrinsic value of options vested or expected to vest </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Aggregate intrinsic value of options exercisable </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeTable" xlink:label="us-gaap_ScheduleOfShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeTable" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ScheduleOfShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeTable" xlink:to="us-gaap_ScheduleOfShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeTable_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ScheduleOfShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeTable_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Schedule of Share-based Compensation, Shares Authorized under Stock Option Plans, by Exercise Price Range [Table] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis" xlink:to="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Exercise Price Range [Axis] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain" xlink:to="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Domain] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Domain] </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_ExercisePriceRangeOneMember" xlink:label="ontx_ExercisePriceRangeOneMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_ExercisePriceRangeOneMember" xlink:to="ontx_ExercisePriceRangeOneMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_ExercisePriceRangeOneMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Represents information about the first range on exercise prices. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_ExercisePriceRangeOneMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Exercise Price Range One [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_ExercisePriceRangeOneMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Exercise Price Range $0.30 - $0.31 </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_ExercisePriceRangeTwoMember" xlink:label="ontx_ExercisePriceRangeTwoMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_ExercisePriceRangeTwoMember" xlink:to="ontx_ExercisePriceRangeTwoMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_ExercisePriceRangeTwoMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Represents information about the second range on exercise prices. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_ExercisePriceRangeTwoMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Exercise Price Range Two [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_ExercisePriceRangeTwoMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Exercise Price Range $3.39 - $3.72 </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_ExercisePriceRangeThreeMember" xlink:label="ontx_ExercisePriceRangeThreeMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_ExercisePriceRangeThreeMember" xlink:to="ontx_ExercisePriceRangeThreeMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_ExercisePriceRangeThreeMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Represents information about the third range on exercise prices. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_ExercisePriceRangeThreeMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Exercise Price Range Three [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_ExercisePriceRangeThreeMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Exercise Price Range $4.34 - $7.05 </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_ExercisePriceRangeFourMember" xlink:label="ontx_ExercisePriceRangeFourMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_ExercisePriceRangeFourMember" xlink:to="ontx_ExercisePriceRangeFourMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_ExercisePriceRangeFourMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Represents information about the fourth range on exercise prices. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_ExercisePriceRangeFourMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Exercise Price Range Four [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_ExercisePriceRangeFourMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Exercise Price Range $16.35 - $97.50 </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_ExercisePriceRangeFiveMember" xlink:label="ontx_ExercisePriceRangeFiveMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_ExercisePriceRangeFiveMember" xlink:to="ontx_ExercisePriceRangeFiveMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_ExercisePriceRangeFiveMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Represents information about the fifth range on exercise prices. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_ExercisePriceRangeFiveMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Exercise Price Range Five [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_ExercisePriceRangeFiveMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Exercise Price Range $222.00 - $225.00 </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_ExercisePriceRangeSixMember" xlink:label="ontx_ExercisePriceRangeSixMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_ExercisePriceRangeSixMember" xlink:to="ontx_ExercisePriceRangeSixMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_ExercisePriceRangeSixMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Represents information about the sixth range on exercise prices. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_ExercisePriceRangeSixMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Exercise Price Range Six [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_ExercisePriceRangeSixMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Exercise Price Range $348.00 - $597.00 </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_ExercisePriceRangeSevenMember" xlink:label="ontx_ExercisePriceRangeSevenMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_ExercisePriceRangeSevenMember" xlink:to="ontx_ExercisePriceRangeSevenMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_ExercisePriceRangeSevenMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Represents information about the seventh range on exercise prices. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_ExercisePriceRangeSevenMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Exercise Price Range Seven [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_ExercisePriceRangeSevenMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Exercise Price Range $651.00 - $1,129.50 </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_ExercisePriceRangeEightMember" xlink:label="ontx_ExercisePriceRangeEightMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_ExercisePriceRangeEightMember" xlink:to="ontx_ExercisePriceRangeEightMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_ExercisePriceRangeEightMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Represents information about the eighth range on exercise prices. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_ExercisePriceRangeEightMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Exercise Price Range Eight [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_ExercisePriceRangeEightMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Exercise Price Range $1,992.00 - $2,268.00 </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_ExercisePriceRangeNineMember" xlink:label="ontx_ExercisePriceRangeNineMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_ExercisePriceRangeNineMember" xlink:to="ontx_ExercisePriceRangeNineMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_ExercisePriceRangeNineMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Represents information about the ninth range on exercise price. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_ExercisePriceRangeNineMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Exercise Price Range Nine [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_ExercisePriceRangeNineMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Exercise Price Range $4,156.50 - $4,371.00 </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="us-gaap_AwardTypeAxis" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_AwardTypeAxis_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AwardTypeAxis_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Award Type [Axis] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share Based Compensation Arrangements By Share Based Payment Award Award Type And Plan Name [Domain] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Equity Award [Domain] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLineItems" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLineItems" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLineItems" xlink:to="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLineItems_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLineItems_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeEndOfPeriodAbstract" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeEndOfPeriodAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeEndOfPeriodAbstract" xlink:to="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeEndOfPeriodAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeEndOfPeriodAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeEndOfPeriodAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-Based Compensation </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit" xlink:to="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Lower Range Limit </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Exercise price, lower range limit (in dollars per share) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit" xlink:to="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Upper Range Limit </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Exercise price, upper range limit (in dollars per share) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions" xlink:to="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Number of Outstanding Options </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Shares (in shares) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions" xlink:to="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Number of Exercisable Options </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Exercisable (in shares) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions" xlink:to="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Stock Options </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Unrecognized compensation expense related to unvested stock options </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" xlink:to="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Weighted-average period for recognizing unrecognized compensation expense related to unvested stock options (in years) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Assumptions used </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Risk-free interest rate (as a percent) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Expected volatility (as a percent) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Expected term (in years) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Expected dividend yield (as a percent) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Weighted average grant date fair value (in dollars per share) </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeitureRate" xlink:label="ontx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeitureRate" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeitureRate" xlink:to="ontx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeitureRate_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeitureRate_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Represents the options forfeiture rate under stock based compensation plan. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeitureRate_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Share Based Compensation Arrangement by Share Based Payment Award Options Forfeiture Rate </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeitureRate_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Annualized forfeiture rate (as a percent) </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax" xlink:label="us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax" xlink:to="us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Revenue from Contract with Customer, Including Assessed Tax </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Revenue </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_LongTermPurchaseCommitmentAmount" xlink:label="us-gaap_LongTermPurchaseCommitmentAmount" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_LongTermPurchaseCommitmentAmount" xlink:to="us-gaap_LongTermPurchaseCommitmentAmount_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_LongTermPurchaseCommitmentAmount_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Committed amount </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_LongTermPurchaseCommitmentAmount_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Committed amount </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_CollaborativeArrangementUpfrontFes" xlink:label="ontx_CollaborativeArrangementUpfrontFes" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_CollaborativeArrangementUpfrontFes" xlink:to="ontx_CollaborativeArrangementUpfrontFes_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_CollaborativeArrangementUpfrontFes_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> The amount of upfront fees paid by the entity under the agreement. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_CollaborativeArrangementUpfrontFes_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Collaborative Arrangement Upfront Fes </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_CollaborativeArrangementUpfrontFes_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Collaborative Arrangement upfront fees </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RevenueFromCollaborativeArrangementExcludingRevenueFromContractWithCustomer" xlink:label="us-gaap_RevenueFromCollaborativeArrangementExcludingRevenueFromContractWithCustomer" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_RevenueFromCollaborativeArrangementExcludingRevenueFromContractWithCustomer" xlink:to="us-gaap_RevenueFromCollaborativeArrangementExcludingRevenueFromContractWithCustomer_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_RevenueFromCollaborativeArrangementExcludingRevenueFromContractWithCustomer_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Revenue from Collaborative Arrangement, Excluding Revenue from Contract with Customer </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_RevenueFromCollaborativeArrangementExcludingRevenueFromContractWithCustomer_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Collaboration revenue </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EscrowDeposit" xlink:label="us-gaap_EscrowDeposit" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_EscrowDeposit" xlink:to="us-gaap_EscrowDeposit_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_EscrowDeposit_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Escrow Deposit </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_EscrowDeposit_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Escrow Deposit </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_CollaborativeArrangementAggregatePotentialMilestonePaymentsBasedOnAnnualNetSales" xlink:label="ontx_CollaborativeArrangementAggregatePotentialMilestonePaymentsBasedOnAnnualNetSales" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_CollaborativeArrangementAggregatePotentialMilestonePaymentsBasedOnAnnualNetSales" xlink:to="ontx_CollaborativeArrangementAggregatePotentialMilestonePaymentsBasedOnAnnualNetSales_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_CollaborativeArrangementAggregatePotentialMilestonePaymentsBasedOnAnnualNetSales_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> The aggregate amount of payments that could be received based on achievement of annual net sales under the arrangement. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_CollaborativeArrangementAggregatePotentialMilestonePaymentsBasedOnAnnualNetSales_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Collaborative Arrangement Aggregate Potential Milestone Payments Based on Annual Net Sales </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_CollaborativeArrangementAggregatePotentialMilestonePaymentsBasedOnAnnualNetSales_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Aggregate potential milestone payments based on annual net sales </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ContractWithCustomerAssetNetCurrent" xlink:label="us-gaap_ContractWithCustomerAssetNetCurrent" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ContractWithCustomerAssetNetCurrent" xlink:to="us-gaap_ContractWithCustomerAssetNetCurrent_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ContractWithCustomerAssetNetCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Contract with Customer, Asset, Net, Current </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:label="us-gaap_ContractWithCustomerAssetNetCurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Receivable recorded </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RedemptionPremium" xlink:label="us-gaap_RedemptionPremium" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_RedemptionPremium" xlink:to="us-gaap_RedemptionPremium_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_RedemptionPremium_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Redemption Premium </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_RedemptionPremium_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Redemption premium </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_ContractWithCustomerLiabilityReversalOfRevenueRecognized" xlink:label="ontx_ContractWithCustomerLiabilityReversalOfRevenueRecognized" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_ContractWithCustomerLiabilityReversalOfRevenueRecognized" xlink:to="ontx_ContractWithCustomerLiabilityReversalOfRevenueRecognized_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_ContractWithCustomerLiabilityReversalOfRevenueRecognized_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Contract with Customer, Liability, Reversal of Revenue Recognized </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_ContractWithCustomerLiabilityReversalOfRevenueRecognized_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Contract with Customer, Liability, Reversal of Revenue Recognized </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_ContractWithCustomerLiabilityReversalOfRevenueRecognized_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Reversal of revenue recognized </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:label="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Schedule of Related Party Transactions, by Related Party [Table] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RelatedPartyTransactionLineItems" xlink:label="us-gaap_RelatedPartyTransactionLineItems" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_RelatedPartyTransactionLineItems_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_RelatedPartyTransactionLineItems_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Related Party Transaction [Line Items] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_RelatedPartyTransactionLineItems_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Related-Party Transactions </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RelatedPartyTransactionExpensesFromTransactionsWithRelatedParty" xlink:label="us-gaap_RelatedPartyTransactionExpensesFromTransactionsWithRelatedParty" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_RelatedPartyTransactionExpensesFromTransactionsWithRelatedParty" xlink:to="us-gaap_RelatedPartyTransactionExpensesFromTransactionsWithRelatedParty_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_RelatedPartyTransactionExpensesFromTransactionsWithRelatedParty_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Related Party Transaction, Expenses from Transactions with Related Party </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_RelatedPartyTransactionExpensesFromTransactionsWithRelatedParty_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Payments to related party </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AccountsPayableRelatedPartiesCurrentAndNoncurrent" xlink:label="us-gaap_AccountsPayableRelatedPartiesCurrentAndNoncurrent" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_AccountsPayableRelatedPartiesCurrentAndNoncurrent" xlink:to="us-gaap_AccountsPayableRelatedPartiesCurrentAndNoncurrent_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AccountsPayableRelatedPartiesCurrentAndNoncurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Accounts Payable, Related Parties </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_AccountsPayableRelatedPartiesCurrentAndNoncurrent_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Amounts due to related party </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_PercentageOfCashFeeOnGrossProceeds" xlink:label="ontx_PercentageOfCashFeeOnGrossProceeds" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_PercentageOfCashFeeOnGrossProceeds" xlink:to="ontx_PercentageOfCashFeeOnGrossProceeds_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_PercentageOfCashFeeOnGrossProceeds_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> The percentage of cash fees on gross proceeds. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_PercentageOfCashFeeOnGrossProceeds_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Percentage of Cash Fee on Gross Proceeds </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_PercentageOfCashFeeOnGrossProceeds_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Percentage of cash fees on gross proceeds </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_PercentageOfManagementFee" xlink:label="ontx_PercentageOfManagementFee" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_PercentageOfManagementFee" xlink:to="ontx_PercentageOfManagementFee_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_PercentageOfManagementFee_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> The percentage of management fee on gross proceeds of the offering and other expenses. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_PercentageOfManagementFee_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Percentage of Management Fee </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_PercentageOfManagementFee_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Percentage of management fees on gross proceeds </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_PaymentOfNonAccountableExpenses" xlink:label="ontx_PaymentOfNonAccountableExpenses" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_PaymentOfNonAccountableExpenses" xlink:to="ontx_PaymentOfNonAccountableExpenses_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_PaymentOfNonAccountableExpenses_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> The amount of non-accountable expenses paid. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_PaymentOfNonAccountableExpenses_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Payment of Non-accountable Expenses </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_PaymentOfClearingFees" xlink:label="ontx_PaymentOfClearingFees" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_PaymentOfClearingFees" xlink:to="ontx_PaymentOfClearingFees_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_PaymentOfClearingFees_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> The amount of clearing fees paid. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_PaymentOfClearingFees_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Payment of Clearing Fees </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_WarrantsIssuedToPurchaseOfCommonStock" xlink:label="ontx_WarrantsIssuedToPurchaseOfCommonStock" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_WarrantsIssuedToPurchaseOfCommonStock" xlink:to="ontx_WarrantsIssuedToPurchaseOfCommonStock_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_WarrantsIssuedToPurchaseOfCommonStock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Warrants issued to purchase of common stock. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_WarrantsIssuedToPurchaseOfCommonStock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Warrants Issued to Purchase of Common Stock </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_WarrantsIssuedToPurchaseOfCommonStock_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Warrants issued to purchase of common stock </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ProceedsFromIssuanceOfWarrants" xlink:label="us-gaap_ProceedsFromIssuanceOfWarrants" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ProceedsFromIssuanceOfWarrants" xlink:to="us-gaap_ProceedsFromIssuanceOfWarrants_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ProceedsFromIssuanceOfWarrants_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Proceeds from Issuance of Warrants </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ProceedsFromIssuanceOfWarrants_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Proceeds from issuance of warrants </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="us-gaap_StatementEquityComponentsAxis" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_StatementEquityComponentsAxis_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_StatementEquityComponentsAxis_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Equity Components [Axis] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="us-gaap_EquityComponentDomain" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_EquityComponentDomain_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_EquityComponentDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Equity Component [Domain] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_EquityComponentDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Equity Component [Domain] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CommonStockMember" xlink:label="us-gaap_CommonStockMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_CommonStockMember" xlink:to="us-gaap_CommonStockMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_CommonStockMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Common Stock [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_CommonStockMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Common Stock </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AdditionalPaidInCapitalMember" xlink:label="us-gaap_AdditionalPaidInCapitalMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_AdditionalPaidInCapitalMember" xlink:to="us-gaap_AdditionalPaidInCapitalMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AdditionalPaidInCapitalMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Additional Paid In Capital [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_AdditionalPaidInCapitalMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Additional Paid in Capital </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RetainedEarningsMember" xlink:label="us-gaap_RetainedEarningsMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_RetainedEarningsMember" xlink:to="us-gaap_RetainedEarningsMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_RetainedEarningsMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Retained Earnings [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_RetainedEarningsMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Accumulated deficit </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember" xlink:label="us-gaap_AccumulatedOtherComprehensiveIncomeMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_AccumulatedOtherComprehensiveIncomeMember" xlink:to="us-gaap_AccumulatedOtherComprehensiveIncomeMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AccumulatedOtherComprehensiveIncomeMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Accumulated Other Comprehensive Income [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_AccumulatedOtherComprehensiveIncomeMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Accumulated other comprehensive loss </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ClassOfWarrantOrRightAxis" xlink:label="us-gaap_ClassOfWarrantOrRightAxis" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ClassOfWarrantOrRightAxis" xlink:to="us-gaap_ClassOfWarrantOrRightAxis_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ClassOfWarrantOrRightAxis_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Class of Warrant or Right [Axis] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ClassOfWarrantOrRightDomain" xlink:label="us-gaap_ClassOfWarrantOrRightDomain" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ClassOfWarrantOrRightDomain" xlink:to="us-gaap_ClassOfWarrantOrRightDomain_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ClassOfWarrantOrRightDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Class Of Warrant Or Right [Domain] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ClassOfWarrantOrRightDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Class of Warrant or Right [Domain] </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_PrefundedWarrantsMember" xlink:label="ontx_PrefundedWarrantsMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_PrefundedWarrantsMember" xlink:to="ontx_PrefundedWarrantsMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_PrefundedWarrantsMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Pre-funded warrants. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_PrefundedWarrantsMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Prefunded Warrants [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_PrefundedWarrantsMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Pre-funded warrants </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="us-gaap_IncomeStatementLocationAxis" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_IncomeStatementLocationAxis" xlink:to="us-gaap_IncomeStatementLocationAxis_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_IncomeStatementLocationAxis_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Income Statement Location [Axis] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="us-gaap_IncomeStatementLocationDomain" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_IncomeStatementLocationDomain" xlink:to="us-gaap_IncomeStatementLocationDomain_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_IncomeStatementLocationDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Income Statement Location [Domain] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_IncomeStatementLocationDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Income Statement Location [Domain] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_GeneralAndAdministrativeExpenseMember" xlink:label="us-gaap_GeneralAndAdministrativeExpenseMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_GeneralAndAdministrativeExpenseMember" xlink:to="us-gaap_GeneralAndAdministrativeExpenseMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_GeneralAndAdministrativeExpenseMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> General And Administrative Expense [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_GeneralAndAdministrativeExpenseMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> General and administrative </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ResearchAndDevelopmentExpenseMember" xlink:label="us-gaap_ResearchAndDevelopmentExpenseMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ResearchAndDevelopmentExpenseMember" xlink:to="us-gaap_ResearchAndDevelopmentExpenseMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ResearchAndDevelopmentExpenseMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Research And Development Expense [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ResearchAndDevelopmentExpenseMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Research and development </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_TypeOfArrangementAxis" xlink:label="us-gaap_TypeOfArrangementAxis" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_TypeOfArrangementAxis" xlink:to="us-gaap_TypeOfArrangementAxis_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_TypeOfArrangementAxis_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Type of Arrangement and Non-arrangement Transactions [Axis] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="us-gaap_ArrangementsAndNonarrangementTransactionsMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:to="us-gaap_ArrangementsAndNonarrangementTransactionsMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ArrangementsAndNonarrangementTransactionsMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Arrangements And Nonarrangement Transactions [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ArrangementsAndNonarrangementTransactionsMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Arrangements and Non-arrangement Transactions [Domain] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CollaborativeArrangementTransactionWithPartyToCollaborativeArrangementMember" xlink:label="us-gaap_CollaborativeArrangementTransactionWithPartyToCollaborativeArrangementMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_CollaborativeArrangementTransactionWithPartyToCollaborativeArrangementMember" xlink:to="us-gaap_CollaborativeArrangementTransactionWithPartyToCollaborativeArrangementMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_CollaborativeArrangementTransactionWithPartyToCollaborativeArrangementMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Collaborative Arrangement Transaction With Party To Collaborative Arrangement [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_CollaborativeArrangementTransactionWithPartyToCollaborativeArrangementMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> License and collaboration agreements </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_CounterpartyNameAxis" xlink:label="srt_CounterpartyNameAxis" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="srt_CounterpartyNameAxis" xlink:to="srt_CounterpartyNameAxis_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="srt_CounterpartyNameAxis_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Counterparty Name [Axis] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="srt_RepurchaseAgreementCounterpartyNameDomain" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="srt_RepurchaseAgreementCounterpartyNameDomain" xlink:to="srt_RepurchaseAgreementCounterpartyNameDomain_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="srt_RepurchaseAgreementCounterpartyNameDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Repurchase Agreement Counterparty Name [Domain] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="srt_RepurchaseAgreementCounterpartyNameDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Counterparty Name [Domain] </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_SymBioPharmaceuticalsLimitedMember" xlink:label="ontx_SymBioPharmaceuticalsLimitedMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_SymBioPharmaceuticalsLimitedMember" xlink:to="ontx_SymBioPharmaceuticalsLimitedMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_SymBioPharmaceuticalsLimitedMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Represents information pertaining to SymBio Pharmaceuticals Limited. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_SymBioPharmaceuticalsLimitedMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Sym Bio Pharmaceuticals Limited [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_SymBioPharmaceuticalsLimitedMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> SymBio </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_ProductOrServiceAxis" xlink:label="srt_ProductOrServiceAxis" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="srt_ProductOrServiceAxis" xlink:to="srt_ProductOrServiceAxis_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="srt_ProductOrServiceAxis_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Products and Services [Axis] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_ProductsAndServicesDomain" xlink:label="srt_ProductsAndServicesDomain" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="srt_ProductsAndServicesDomain" xlink:to="srt_ProductsAndServicesDomain_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="srt_ProductsAndServicesDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Products And Services [Domain] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="srt_ProductsAndServicesDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Products and Services [Domain] </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_UpfrontLicenseFeeRecognitionOverTimeMember" xlink:label="ontx_UpfrontLicenseFeeRecognitionOverTimeMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_UpfrontLicenseFeeRecognitionOverTimeMember" xlink:to="ontx_UpfrontLicenseFeeRecognitionOverTimeMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_UpfrontLicenseFeeRecognitionOverTimeMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Represents the information relating to Upfront license fee recognition over time </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_UpfrontLicenseFeeRecognitionOverTimeMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Upfront License Fee Recognition Over Time [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_UpfrontLicenseFeeRecognitionOverTimeMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Upfront license fee recognition over time </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_SuppliesAndOtherMember" xlink:label="ontx_SuppliesAndOtherMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_SuppliesAndOtherMember" xlink:to="ontx_SuppliesAndOtherMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_SuppliesAndOtherMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> It represents Supplies and other member. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_SuppliesAndOtherMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Supplies And Other [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_SuppliesAndOtherMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Supplies and other </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_NonTradableWarrantsExpiringJuly2021LiabilityMember" xlink:label="ontx_NonTradableWarrantsExpiringJuly2021LiabilityMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_NonTradableWarrantsExpiringJuly2021LiabilityMember" xlink:to="ontx_NonTradableWarrantsExpiringJuly2021LiabilityMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_NonTradableWarrantsExpiringJuly2021LiabilityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non-tradable warrants expiring July 2021, and classified as a liability. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_NonTradableWarrantsExpiringJuly2021LiabilityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non Tradable Warrants Expiring July2021 Liability [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_NonTradableWarrantsExpiringJuly2021LiabilityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non-tradable warrants with exercise price 172.50 Liability </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_TradableWarrantsExpiringJuly2021LiabilityMember" xlink:label="ontx_TradableWarrantsExpiringJuly2021LiabilityMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_TradableWarrantsExpiringJuly2021LiabilityMember" xlink:to="ontx_TradableWarrantsExpiringJuly2021LiabilityMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_TradableWarrantsExpiringJuly2021LiabilityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Represents information pertaining to tradable warrants expiring July 2021 liability. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_TradableWarrantsExpiringJuly2021LiabilityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Tradable Warrants Expiring July2021 Liability [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_TradableWarrantsExpiringJuly2021LiabilityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Tradable warrants with exercise price 73.80 Liability </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_NonTradablePreFundedWarrantsExpiringJuly2023EquityMember" xlink:label="ontx_NonTradablePreFundedWarrantsExpiringJuly2023EquityMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_NonTradablePreFundedWarrantsExpiringJuly2023EquityMember" xlink:to="ontx_NonTradablePreFundedWarrantsExpiringJuly2023EquityMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_NonTradablePreFundedWarrantsExpiringJuly2023EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non-tradable pre-funded warrants expiring July 2023, and classified as equity. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_NonTradablePreFundedWarrantsExpiringJuly2023EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non Tradable Pre Funded Warrants Expiring July2023 Equity [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_NonTradablePreFundedWarrantsExpiringJuly2023EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non-tradable warrants with exercise price 0.15 Equity </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_NonTradableWarrantsWithExercisePrice1.60EquityMember" xlink:label="ontx_NonTradableWarrantsWithExercisePrice1.60EquityMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_NonTradableWarrantsWithExercisePrice1.60EquityMember" xlink:to="ontx_NonTradableWarrantsWithExercisePrice1.60EquityMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_NonTradableWarrantsWithExercisePrice1.60EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Representing information pertaining to non-tradable warrants with exercise price of 1.60 and classified as equity during the period. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_NonTradableWarrantsWithExercisePrice1.60EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non Tradable Warrants With Exercise Price1.60 Equity [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_NonTradableWarrantsWithExercisePrice1.60EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non-tradable warrants with exercise price 1.60 Equity </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_TradableWarrantsWithExercisePrice14.10EquityMember" xlink:label="ontx_TradableWarrantsWithExercisePrice14.10EquityMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_TradableWarrantsWithExercisePrice14.10EquityMember" xlink:to="ontx_TradableWarrantsWithExercisePrice14.10EquityMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_TradableWarrantsWithExercisePrice14.10EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Representing information pertaining to non - tradable warrants with exercise price of 14.10 and classified as equity during the period. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_TradableWarrantsWithExercisePrice14.10EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Tradable Warrants With Exercise Price14.10 Equity [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_TradableWarrantsWithExercisePrice14.10EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non-tradable warrants with exercise price 14.10 Equity </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_NonTradableWarrantsWithExercisePrice21.15EquityMember" xlink:label="ontx_NonTradableWarrantsWithExercisePrice21.15EquityMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_NonTradableWarrantsWithExercisePrice21.15EquityMember" xlink:to="ontx_NonTradableWarrantsWithExercisePrice21.15EquityMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_NonTradableWarrantsWithExercisePrice21.15EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Representing information pertaining to non - tradable warrants with exercise price of 21.15 and classified as equity during the period. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_NonTradableWarrantsWithExercisePrice21.15EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non Tradable Warrants With Exercise Price21.15 Equity [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_NonTradableWarrantsWithExercisePrice21.15EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non-tradable warrants with exercise price 21.15 Equity </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_NonTradableWarrantsWithExercisePrice7.7895EquityMember" xlink:label="ontx_NonTradableWarrantsWithExercisePrice7.7895EquityMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_NonTradableWarrantsWithExercisePrice7.7895EquityMember" xlink:to="ontx_NonTradableWarrantsWithExercisePrice7.7895EquityMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_NonTradableWarrantsWithExercisePrice7.7895EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Representing information pertaining to non - tradable warrants with exercise price of 7.7895 and classified as equity during the period. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_NonTradableWarrantsWithExercisePrice7.7895EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non Tradable Warrants With Exercise Price7.7895 Equity [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_NonTradableWarrantsWithExercisePrice7.7895EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non-tradable warrants with exercise price 7.7895 Equity </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_NonTradablePreFundedWarrantsWithExercisePrice0.15EquityMember" xlink:label="ontx_NonTradablePreFundedWarrantsWithExercisePrice0.15EquityMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_NonTradablePreFundedWarrantsWithExercisePrice0.15EquityMember" xlink:to="ontx_NonTradablePreFundedWarrantsWithExercisePrice0.15EquityMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_NonTradablePreFundedWarrantsWithExercisePrice0.15EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Representing information pertaining to non-tradable pre-funded warrants with exercise price of 0.15 and classified as equity during the period. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_NonTradablePreFundedWarrantsWithExercisePrice0.15EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non Tradable Pre Funded Warrants With Exercise Price0.15 Equity [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_NonTradablePreFundedWarrantsWithExercisePrice0.15EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non-tradable pre-funded warrants with exercise Price 0.15 Equity </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_NonTradableWarrantsWithExercisePrice1.6000EquityMember" xlink:label="ontx_NonTradableWarrantsWithExercisePrice1.6000EquityMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_NonTradableWarrantsWithExercisePrice1.6000EquityMember" xlink:to="ontx_NonTradableWarrantsWithExercisePrice1.6000EquityMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_NonTradableWarrantsWithExercisePrice1.6000EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Representing information pertaining to non-tradable warrants with exercise price of 1.6000 and classified as equity during the period. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_NonTradableWarrantsWithExercisePrice1.6000EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non Tradable Warrants With Exercise Price1.6000 Equity [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_NonTradableWarrantsWithExercisePrice1.6000EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non-tradable warrants with exercise price 1.6000 Equity </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_NonTradablePreFundedWarrantsWithExercisePrice0.15EquityOneMember" xlink:label="ontx_NonTradablePreFundedWarrantsWithExercisePrice0.15EquityOneMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_NonTradablePreFundedWarrantsWithExercisePrice0.15EquityOneMember" xlink:to="ontx_NonTradablePreFundedWarrantsWithExercisePrice0.15EquityOneMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_NonTradablePreFundedWarrantsWithExercisePrice0.15EquityOneMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Representing information pertaining to non-tradable pre-funded warrants with exercise price of 0.15 -one and classified as equity during the period. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_NonTradablePreFundedWarrantsWithExercisePrice0.15EquityOneMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non Tradable Pre Funded Warrants With Exercise Price0.15 Equity One [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_NonTradablePreFundedWarrantsWithExercisePrice0.15EquityOneMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non-tradable pre-funded warrants with exercise Price 0.15 Equity1 </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_NonTradableWarrantsWithExercisePrice2.00EquityMember" xlink:label="ontx_NonTradableWarrantsWithExercisePrice2.00EquityMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_NonTradableWarrantsWithExercisePrice2.00EquityMember" xlink:to="ontx_NonTradableWarrantsWithExercisePrice2.00EquityMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_NonTradableWarrantsWithExercisePrice2.00EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Representing information pertaining to non - tradable warrants with exercise price of 2.00 and classified as equity during the period. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_NonTradableWarrantsWithExercisePrice2.00EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non Tradable Warrants With Exercise Price2.00 Equity [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_NonTradableWarrantsWithExercisePrice2.00EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non-tradable warrants with exercise price 2.00 Equity </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_NonTradablePreFundedWarrantsWithExercisePrice0.0001EquityMember" xlink:label="ontx_NonTradablePreFundedWarrantsWithExercisePrice0.0001EquityMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_NonTradablePreFundedWarrantsWithExercisePrice0.0001EquityMember" xlink:to="ontx_NonTradablePreFundedWarrantsWithExercisePrice0.0001EquityMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_NonTradablePreFundedWarrantsWithExercisePrice0.0001EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Representing information pertaining to non-tradable pre-funded warrants with exercise price of 0.0001 and classified as equity during the period. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_NonTradablePreFundedWarrantsWithExercisePrice0.0001EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non Tradable Pre Funded Warrants With Exercise Price0.0001 Equity [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_NonTradablePreFundedWarrantsWithExercisePrice0.0001EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non-tradable pre-funded warrants with exercise Price 0.0001 Equity </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_NonTradableWarrantsWithExercisePrice0.20EquityMember" xlink:label="ontx_NonTradableWarrantsWithExercisePrice0.20EquityMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_NonTradableWarrantsWithExercisePrice0.20EquityMember" xlink:to="ontx_NonTradableWarrantsWithExercisePrice0.20EquityMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_NonTradableWarrantsWithExercisePrice0.20EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Representing information pertaining to non-tradable warrants with exercise price of 0.20 and classified as equity during the period. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_NonTradableWarrantsWithExercisePrice0.20EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non Tradable Warrants With Exercise Price0.20 Equity [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_NonTradableWarrantsWithExercisePrice0.20EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non-tradable warrants with exercise price 0.20 Equity </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_NonTradableWarrantsWithExercisePrice0.250EquityMember" xlink:label="ontx_NonTradableWarrantsWithExercisePrice0.250EquityMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_NonTradableWarrantsWithExercisePrice0.250EquityMember" xlink:to="ontx_NonTradableWarrantsWithExercisePrice0.250EquityMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_NonTradableWarrantsWithExercisePrice0.250EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Representing information pertaining to non-tradable warrants with exercise price of 0.250 and classified as equity during the period. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_NonTradableWarrantsWithExercisePrice0.250EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non Tradable Warrants With Exercise Price0.250 Equity [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_NonTradableWarrantsWithExercisePrice0.250EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non-tradable warrants with exercise price 0.250 Equity </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_NonTradableWarrantsWithExercisePrice0.287EquityMember" xlink:label="ontx_NonTradableWarrantsWithExercisePrice0.287EquityMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_NonTradableWarrantsWithExercisePrice0.287EquityMember" xlink:to="ontx_NonTradableWarrantsWithExercisePrice0.287EquityMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_NonTradableWarrantsWithExercisePrice0.287EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Representing information pertaining to non-tradable warrants with exercise price of 0.287 and classified as equity during the period. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_NonTradableWarrantsWithExercisePrice0.287EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non Tradable Warrants With Exercise Price0.287 Equity [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_NonTradableWarrantsWithExercisePrice0.287EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non-tradable warrants with exercise price 0.287 Equity </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_NonTradableWarrantsWithExercisePrice0.43625EquityMember" xlink:label="ontx_NonTradableWarrantsWithExercisePrice0.43625EquityMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_NonTradableWarrantsWithExercisePrice0.43625EquityMember" xlink:to="ontx_NonTradableWarrantsWithExercisePrice0.43625EquityMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_NonTradableWarrantsWithExercisePrice0.43625EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Representing information pertaining to non-tradable warrants with exercise price of 0.43625 and classified as equity during the period. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_NonTradableWarrantsWithExercisePrice0.43625EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non Tradable Warrants With Exercise Price0.43625 Equity [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_NonTradableWarrantsWithExercisePrice0.43625EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non-tradable warrants with exercise price 0.43625 Equity </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_NonTradableWarrantsWithExercisePrice0.298EquityMember" xlink:label="ontx_NonTradableWarrantsWithExercisePrice0.298EquityMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_NonTradableWarrantsWithExercisePrice0.298EquityMember" xlink:to="ontx_NonTradableWarrantsWithExercisePrice0.298EquityMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_NonTradableWarrantsWithExercisePrice0.298EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Representing information pertaining to non-tradable warrants with exercise price of 0.298 and classified as equity during the period. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_NonTradableWarrantsWithExercisePrice0.298EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non Tradable Warrants With Exercise Price0.298 Equity [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_NonTradableWarrantsWithExercisePrice0.298EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non-tradable warrants with exercise price 0.298 Equity </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_NonTradableWarrantsWithExercisePrice0.45030EquityMember" xlink:label="ontx_NonTradableWarrantsWithExercisePrice0.45030EquityMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_NonTradableWarrantsWithExercisePrice0.45030EquityMember" xlink:to="ontx_NonTradableWarrantsWithExercisePrice0.45030EquityMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_NonTradableWarrantsWithExercisePrice0.45030EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Representing information pertaining to non-tradable warrants with exercise price of 0.45030 and classified as equity during the period. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_NonTradableWarrantsWithExercisePrice0.45030EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non Tradable Warrants With Exercise Price0.45030 Equity [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_NonTradableWarrantsWithExercisePrice0.45030EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non-tradable warrants with exercise price 0.45030 Equity </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_NonTradableWarrantsWithExercisePrice0.45190EquityMember" xlink:label="ontx_NonTradableWarrantsWithExercisePrice0.45190EquityMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_NonTradableWarrantsWithExercisePrice0.45190EquityMember" xlink:to="ontx_NonTradableWarrantsWithExercisePrice0.45190EquityMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_NonTradableWarrantsWithExercisePrice0.45190EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Represents information pertaining to Non-tradable Warrants with Exercise Price 0.45190 of Equity during the period. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_NonTradableWarrantsWithExercisePrice0.45190EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non Tradable Warrants With Exercise Price0.45190 Equity [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_NonTradableWarrantsWithExercisePrice0.45190EquityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Non-tradable Warrants with Exercise Price 0.45190 Equity </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_CertainInstitutionalAndAccreditedInvestorsMember" xlink:label="ontx_CertainInstitutionalAndAccreditedInvestorsMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_CertainInstitutionalAndAccreditedInvestorsMember" xlink:to="ontx_CertainInstitutionalAndAccreditedInvestorsMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_CertainInstitutionalAndAccreditedInvestorsMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> This member represents certain institutional and accredited investors. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_CertainInstitutionalAndAccreditedInvestorsMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Certain Institutional And Accredited Investors [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_CertainInstitutionalAndAccreditedInvestorsMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Certain institutional and accredited investors </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_MeasurementInputTypeAxis" xlink:label="us-gaap_MeasurementInputTypeAxis" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_MeasurementInputTypeAxis" xlink:to="us-gaap_MeasurementInputTypeAxis_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_MeasurementInputTypeAxis_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Measurement Input Type [Axis] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_MeasurementInputTypeDomain" xlink:label="us-gaap_MeasurementInputTypeDomain" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_MeasurementInputTypeDomain" xlink:to="us-gaap_MeasurementInputTypeDomain_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_MeasurementInputTypeDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Measurement Input Type [Domain] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_MeasurementInputTypeDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Measurement Input Type [Domain] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_MeasurementInputRiskFreeInterestRateMember" xlink:label="us-gaap_MeasurementInputRiskFreeInterestRateMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_MeasurementInputRiskFreeInterestRateMember" xlink:to="us-gaap_MeasurementInputRiskFreeInterestRateMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_MeasurementInputRiskFreeInterestRateMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Measurement Input Risk Free Interest Rate [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_MeasurementInputRiskFreeInterestRateMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Risk-free interest rate </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_MeasurementInputPriceVolatilityMember" xlink:label="us-gaap_MeasurementInputPriceVolatilityMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_MeasurementInputPriceVolatilityMember" xlink:to="us-gaap_MeasurementInputPriceVolatilityMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_MeasurementInputPriceVolatilityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Measurement Input Price Volatility [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_MeasurementInputPriceVolatilityMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Expected volatility </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_MeasurementInputExpectedTermMember" xlink:label="us-gaap_MeasurementInputExpectedTermMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_MeasurementInputExpectedTermMember" xlink:to="us-gaap_MeasurementInputExpectedTermMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_MeasurementInputExpectedTermMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Measurement Input Expected Term [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_MeasurementInputExpectedTermMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Expected term </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_MeasurementInputExpectedDividendRateMember" xlink:label="us-gaap_MeasurementInputExpectedDividendRateMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_MeasurementInputExpectedDividendRateMember" xlink:to="us-gaap_MeasurementInputExpectedDividendRateMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_MeasurementInputExpectedDividendRateMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Measurement Input Expected Dividend Rate [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_MeasurementInputExpectedDividendRateMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Expected dividend yield </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueByMeasurementFrequencyAxis" xlink:label="us-gaap_FairValueByMeasurementFrequencyAxis" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueByMeasurementFrequencyAxis" xlink:to="us-gaap_FairValueByMeasurementFrequencyAxis_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueByMeasurementFrequencyAxis_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Measurement Frequency [Axis] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="us-gaap_FairValueMeasurementFrequencyDomain" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueMeasurementFrequencyDomain" xlink:to="us-gaap_FairValueMeasurementFrequencyDomain_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueMeasurementFrequencyDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Fair Value Measurement Frequency [Domain] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_FairValueMeasurementFrequencyDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Fair Value, Measurement Frequency [Domain] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueMeasurementsRecurringMember" xlink:label="us-gaap_FairValueMeasurementsRecurringMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueMeasurementsRecurringMember" xlink:to="us-gaap_FairValueMeasurementsRecurringMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueMeasurementsRecurringMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Fair Value Measurements Recurring [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_FairValueMeasurementsRecurringMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Recurring basis </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="us-gaap_FairValueByFairValueHierarchyLevelAxis" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:to="us-gaap_FairValueByFairValueHierarchyLevelAxis_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueByFairValueHierarchyLevelAxis_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Fair Value, Hierarchy [Axis] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:to="us-gaap_FairValueMeasurementsFairValueHierarchyDomain_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Fair Value Measurements Fair Value Hierarchy [Domain] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Fair Value Hierarchy [Domain] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueInputsLevel1Member" xlink:label="us-gaap_FairValueInputsLevel1Member" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueInputsLevel1Member" xlink:to="us-gaap_FairValueInputsLevel1Member_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueInputsLevel1Member_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Fair Value Inputs Level1 [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_FairValueInputsLevel1Member_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Level 1 </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_TradableWarrantsMember" xlink:label="ontx_TradableWarrantsMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_TradableWarrantsMember" xlink:to="ontx_TradableWarrantsMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_TradableWarrantsMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Tradable warrants. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_TradableWarrantsMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Tradable Warrants [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_TradableWarrantsMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Tradable warrants liability </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PlanNameAxis" xlink:label="us-gaap_PlanNameAxis" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_PlanNameAxis" xlink:to="us-gaap_PlanNameAxis_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_PlanNameAxis_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Plan Name [Axis] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PlanNameDomain" xlink:label="us-gaap_PlanNameDomain" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_PlanNameDomain" xlink:to="us-gaap_PlanNameDomain_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_PlanNameDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Plan Name [Domain] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_PlanNameDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Plan Name [Domain] </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_EquityCompensationPlan2013Member" xlink:label="ontx_EquityCompensationPlan2013Member" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_EquityCompensationPlan2013Member" xlink:to="ontx_EquityCompensationPlan2013Member_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_EquityCompensationPlan2013Member_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Represents the information pertaining to 2013 Equity Compensation Plan. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_EquityCompensationPlan2013Member_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Equity Compensation Plan2013 [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_EquityCompensationPlan2013Member_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> 2013 Plan </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_OmnibusIncentiveCompensationPlan2018Member" xlink:label="ontx_OmnibusIncentiveCompensationPlan2018Member" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_OmnibusIncentiveCompensationPlan2018Member" xlink:to="ontx_OmnibusIncentiveCompensationPlan2018Member_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_OmnibusIncentiveCompensationPlan2018Member_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Represents the information pertaining to 2018 Omnibus Incentive Compensation Plan. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_OmnibusIncentiveCompensationPlan2018Member_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Omnibus Incentive Compensation Plan2018 [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_OmnibusIncentiveCompensationPlan2018Member_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> 2018 Plan </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_LicenseMember" xlink:label="us-gaap_LicenseMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_LicenseMember" xlink:to="us-gaap_LicenseMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_LicenseMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> License [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_LicenseMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Research agreements </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_HanxBiopharmaceuticalsIncMember" xlink:label="ontx_HanxBiopharmaceuticalsIncMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_HanxBiopharmaceuticalsIncMember" xlink:to="ontx_HanxBiopharmaceuticalsIncMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_HanxBiopharmaceuticalsIncMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Represents information pertaining to HanX Biopharmaceuticals, Inc. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_HanxBiopharmaceuticalsIncMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Hanx Biopharmaceuticals Inc [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_HanxBiopharmaceuticalsIncMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> HanX </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_BalanceSheetLocationAxis" xlink:label="us-gaap_BalanceSheetLocationAxis" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_BalanceSheetLocationAxis" xlink:to="us-gaap_BalanceSheetLocationAxis_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_BalanceSheetLocationAxis_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Balance Sheet Location [Axis] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="us-gaap_BalanceSheetLocationDomain" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_BalanceSheetLocationDomain" xlink:to="us-gaap_BalanceSheetLocationDomain_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_BalanceSheetLocationDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Balance Sheet Location [Domain] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_BalanceSheetLocationDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Balance Sheet Location [Domain] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OtherCurrentAssetsMember" xlink:label="us-gaap_OtherCurrentAssetsMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OtherCurrentAssetsMember" xlink:to="us-gaap_OtherCurrentAssetsMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OtherCurrentAssetsMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Other Current Assets [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_OtherCurrentAssetsMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Other Current Assets [Member] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Related Party [Axis] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RelatedPartyDomain" xlink:label="us-gaap_RelatedPartyDomain" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_RelatedPartyDomain" xlink:to="us-gaap_RelatedPartyDomain_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_RelatedPartyDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Related Party [Domain] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_RelatedPartyDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Related Party [Domain] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_DirectorMember" xlink:label="srt_DirectorMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="srt_DirectorMember" xlink:to="srt_DirectorMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="srt_DirectorMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Director [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="srt_DirectorMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Member of board of directors </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_MajorityShareholderMember" xlink:label="us-gaap_MajorityShareholderMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_MajorityShareholderMember" xlink:to="us-gaap_MajorityShareholderMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_MajorityShareholderMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Majority Shareholder [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_MajorityShareholderMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Significant shareholder </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SubsidiarySaleOfStockAxis" xlink:label="us-gaap_SubsidiarySaleOfStockAxis" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_SubsidiarySaleOfStockAxis" xlink:to="us-gaap_SubsidiarySaleOfStockAxis_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_SubsidiarySaleOfStockAxis_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Sale of Stock [Axis] </label>
<loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:to="us-gaap_SaleOfStockNameOfTransactionDomain_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Sale Of Stock Name Of Transaction [Domain] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Sale of Stock [Domain] </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_SecondaryPublicOfferingMember" xlink:label="ontx_SecondaryPublicOfferingMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_SecondaryPublicOfferingMember" xlink:to="ontx_SecondaryPublicOfferingMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_SecondaryPublicOfferingMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Represents information pertaining to Public offering subsequent to initial public offering. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_SecondaryPublicOfferingMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Secondary Public Offering [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_SecondaryPublicOfferingMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> The Offering </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_H.c.WainwrightCoLlcMember" xlink:label="ontx_H.c.WainwrightCoLlcMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_H.c.WainwrightCoLlcMember" xlink:to="ontx_H.c.WainwrightCoLlcMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_H.c.WainwrightCoLlcMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Represents the information pertaining to H.C. Wainwright & Co., LLC, with respect to the issuance and sale in an underwritten public offering by the Company. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_H.c.WainwrightCoLlcMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> H.c. Wainwright Co Llc [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_H.c.WainwrightCoLlcMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> HCW </label>
<loc xlink:type="locator" xlink:href="ontx-20200331.xsd#ontx_CommonStockWarrantsMember" xlink:label="ontx_CommonStockWarrantsMember" xmlns="http://www.xbrl.org/2003/linkbase"/>
<labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ontx_CommonStockWarrantsMember" xlink:to="ontx_CommonStockWarrantsMember_lbl" xmlns="http://www.xbrl.org/2003/linkbase"/>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="ontx_CommonStockWarrantsMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Represents information pertaining to common stock warrants. </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="ontx_CommonStockWarrantsMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Common Stock Warrants [Member] </label>
<label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="ontx_CommonStockWarrantsMember_lbl" xml:lang="en-US" xmlns="http://www.xbrl.org/2003/linkbase"> Common Stock Warrants </label>
</link:labelLink>
</link:linkbase>

3 Subsequent Filings that Reference this Filing

  As Of               Filer                 Filing    For·On·As Docs:Size             Issuer                      Filing Agent

 2/11/21  Onconova Therapeutics, Inc.       424B5                  1:429K                                   Toppan Merrill/FA
 2/10/21  Onconova Therapeutics, Inc.       424B5                  1:431K                                   Toppan Merrill/FA
 1/08/21  Onconova Therapeutics, Inc.       424B5                  1:292K                                   Toppan Merrill/FA
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