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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2016-10-01</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001567226</rptOwnerCik>
            <rptOwnerName>Little Thomas Mitchell</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O MARATHON OIL CORPORATION</rptOwnerStreet1>
            <rptOwnerStreet2>5555 SAN FELIPE STREET</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>Executive VP - Operations</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Marathon Oil Corporation Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2016-10-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>38536</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>168428</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Restricted stock granted under Marathon Oil Corporation 2016 Incentive Compensation Plan, which vests in cumulative annual installments of 12,845, 12,845 and 12,846 shares on October 1, 2017, 2018 and 2019, respectively.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Sam A. Mazzu, Attorney-in-Fact for Thomas Mitchell Little</signatureName>
        <signatureDate>2016-10-04</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0306
4
2016-10-01
0
0000101778
MARATHON OIL CORP
MRO
0001567226
Little Thomas Mitchell
C/O MARATHON OIL CORPORATION
5555 SAN FELIPE STREET
HOUSTON
TX
77056
0
1
0
0
Executive VP - Operations
Marathon Oil Corporation Common Stock
2016-10-01
4
A
0
38536
0
A
168428
D
Restricted stock granted under Marathon Oil Corporation 2016 Incentive Compensation Plan, which vests in cumulative annual installments of 12,845, 12,845 and 12,846 shares on October 1, 2017, 2018 and 2019, respectively.
Sam A. Mazzu, Attorney-in-Fact for Thomas Mitchell Little
2016-10-04