<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2020-04-24</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000101778</issuerCik>
        <issuerName>MARATHON OIL CORP</issuerName>
        <issuerTradingSymbol>MRO</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001330746</rptOwnerCik>
            <rptOwnerName>Hedgebeth Reginald D</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O MARATHON OIL CORPORATION</rptOwnerStreet1>
            <rptOwnerStreet2>5555 SAN FELIPE ST.</rptOwnerStreet2>
            <rptOwnerCity>HOUSTON</rptOwnerCity>
            <rptOwnerState>TX</rptOwnerState>
            <rptOwnerZipCode>77056</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>see remarks</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2020-04-24</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1560</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>4.93</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>148064</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2020-04-24</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>4677</value>
                    <footnoteId id="F2"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>4.93</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>143387</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Shares withheld for taxes for the restricted stock granted under Marathon Oil Corporation's 2016 Incentive Compensation Plan on 4/26/2017, which vested in three equal, annual installments beginning one year from the date of grant.</footnote>
        <footnote id="F2">Shares withheld for taxes for the restricted stock granted under Marathon Oil Corporation's 2016 Incentive Compensation Plan on 4/26/2017, which vested in full three years from the date of grant.</footnote>
    </footnotes>

    <remarks>Executive Vice President, General Counsel and Chief Administrative Officer</remarks>

    <ownerSignature>
        <signatureName>Anna Jones, Attorney-in-Fact for Reginald D. Hedgebeth</signatureName>
        <signatureDate>2020-04-27</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0306
4
2020-04-24
0
0000101778
MARATHON OIL CORP
MRO
0001330746
Hedgebeth Reginald D
C/O MARATHON OIL CORPORATION
5555 SAN FELIPE ST.
HOUSTON
TX
77056
0
1
0
0
see remarks
Common Stock
2020-04-24
4
F
0
1560
4.93
D
148064
D
Common Stock
2020-04-24
4
F
0
4677
4.93
D
143387
D
Shares withheld for taxes for the restricted stock granted under Marathon Oil Corporation's 2016 Incentive Compensation Plan on 4/26/2017, which vested in three equal, annual installments beginning one year from the date of grant.
Shares withheld for taxes for the restricted stock granted under Marathon Oil Corporation's 2016 Incentive Compensation Plan on 4/26/2017, which vested in full three years from the date of grant.
Executive Vice President, General Counsel and Chief Administrative Officer
Anna Jones, Attorney-in-Fact for Reginald D. Hedgebeth
2020-04-27