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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2021-03-01</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001604594</rptOwnerCik>
            <rptOwnerName>Donadio Marcela E</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>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>2021-03-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>13392.857</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>79812.301</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Non-Retainer Annual Director Stock Unit Award granted under Marathon Oil Corporation 2019 Incentive Compensation Plan. The annual non-retainer stock based compensation represented by these common stock units, along with their accrued dividend equivalent rights, are payable in shares of common stock upon the reporting person's departure from the Board of Directors.</footnote>
        <footnote id="F2">Includes dividends of 705.397 shares previously not reported pursuant to Rule 16a-11.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Anna Jones, Attorney-in-Fact for Marcela E. Donadio</signatureName>
        <signatureDate>2021-03-03</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0306
4
2021-03-01
0
0000101778
MARATHON OIL CORP
MRO
0001604594
Donadio Marcela E
C/O MARATHON OIL CORPORATION
5555 SAN FELIPE ST.
HOUSTON
TX
77056
1
0
0
0
Common Stock
2021-03-01
4
A
0
13392.857
0
A
79812.301
D
Non-Retainer Annual Director Stock Unit Award granted under Marathon Oil Corporation 2019 Incentive Compensation Plan. The annual non-retainer stock based compensation represented by these common stock units, along with their accrued dividend equivalent rights, are payable in shares of common stock upon the reporting person's departure from the Board of Directors.
Includes dividends of 705.397 shares previously not reported pursuant to Rule 16a-11.
Anna Jones, Attorney-in-Fact for Marcela E. Donadio
2021-03-03