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<?xml version="1.0" encoding="windows-1252"?> |
<ownershipDocument> |
| <schemaVersion> X0306 </schemaVersion> |
| <documentType> 4 </documentType> |
| <periodOfReport> 2020-04-30 </periodOfReport> |
| <notSubjectToSection16> 0 </notSubjectToSection16> |
| <issuer> |
| | <issuerCik> 0001553023 </issuerCik> |
| | <issuerName> CyrusOne Inc. </issuerName> |
| | <issuerTradingSymbol> CONE </issuerTradingSymbol> |
| | </issuer> |
| <reportingOwner> |
| | <reportingOwnerId> |
| | | <rptOwnerCik> 0001386671 </rptOwnerCik> |
| | | <rptOwnerName> Wentworth Lynn A </rptOwnerName> |
| | | </reportingOwnerId> |
| | <reportingOwnerAddress> |
| | | <rptOwnerStreet1> C/O CYRUSONE INC. </rptOwnerStreet1> |
| | | <rptOwnerStreet2> 2850 N HARWOOD ST, SUITE 2200 </rptOwnerStreet2> |
| | | <rptOwnerCity> DALLAS </rptOwnerCity> |
| | | <rptOwnerState> TX </rptOwnerState> |
| | | <rptOwnerZipCode> 75201-2640 </rptOwnerZipCode> |
| | | <rptOwnerStateDescription></rptOwnerStateDescription> |
| | | </reportingOwnerAddress> |
| | <reportingOwnerRelationship> |
| | | <isDirector> 1 </isDirector> |
| | | <isOfficer> 0 </isOfficer> |
| | | <isTenPercentOwner> 0 </isTenPercentOwner> |
| | | <isOther> 0 </isOther> |
| | | <officerTitle></officerTitle> |
| | | <otherText></otherText> |
| | | </reportingOwnerRelationship> |
| | </reportingOwner> |
| <nonDerivativeTable> |
| | <nonDerivativeTransaction> |
| | | <securityTitle> |
| <value> Common Stock </value> |
| </securityTitle> |
| | | <transactionDate> |
| <value> 2020-04-30 </value> |
| </transactionDate> |
| | | <transactionCoding> |
| <transactionFormType> 4 </transactionFormType> |
| <transactionCode> A </transactionCode> |
| <equitySwapInvolved> 0 </equitySwapInvolved> |
| </transactionCoding> |
| | | <transactionAmounts> |
| <transactionShares> |
| <value> 2139 </value> |
| <footnoteId id="F1"/> |
| </transactionShares> |
| <transactionPricePerShare> |
| <value> 0 </value> |
| </transactionPricePerShare> |
| <transactionAcquiredDisposedCode> |
| <value> A </value> |
| </transactionAcquiredDisposedCode> |
| </transactionAmounts> |
| | | <postTransactionAmounts> |
| <sharesOwnedFollowingTransaction> |
| <value> 22790 </value> |
| </sharesOwnedFollowingTransaction> |
| </postTransactionAmounts> |
| | | <ownershipNature> |
| <directOrIndirectOwnership> |
| <value> D </value> |
| </directOrIndirectOwnership> |
| </ownershipNature> |
| | | </nonDerivativeTransaction> |
| | </nonDerivativeTable> |
| <derivativeTable></derivativeTable> |
| <footnotes> |
| | <footnote id="F1"> Annual equity retainer grant of restricted stock. The restricted stock vests on the earlier of the the first anniversary of the grant date or the date immediately prior to the next annual meeting of stockholders. </footnote> |
| | </footnotes> |
| <remarks></remarks> |
| <ownerSignature> |
| | <signatureName> /s/ Robert M. Jackson, Attorney-in-Fact </signatureName> |
| | <signatureDate> 2020-05-04 </signatureDate> |
| | </ownerSignature> |
| </ownershipDocument> |