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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2017-01-03</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001535486</rptOwnerCik>
            <rptOwnerName>Banister Gaurdie E. JR.</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>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Marathon Oil Corporation Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2017-01-03</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>9853.604</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>26610.302</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Marathon Oil Corporation Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>7600</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Banister Resources Ltd.</value>
                    <footnoteId id="F3"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Non-Retainer Annual Director Stock Unit Award granted under Marathon Oil Corporation 2016 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 238.849 shares previously not reported pursuant to Rule 16a-11.</footnote>
        <footnote id="F3">These shares are held by Banister Resources Ltd., a limited partnership of which Banister Management Trust (the &quot;Trust&quot;) is the general partner, and the reporting person and his spouse are the co-trustees of the Trust.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Kim Warnica, Attorney-in-Fact for Gaurdie E. Banister, Jr.</signatureName>
        <signatureDate>2017-01-04</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0306
4
2017-01-03
0
0000101778
MARATHON OIL CORP
MRO
0001535486
Banister Gaurdie E. JR.
C/O MARATHON OIL CORPORATION
5555 SAN FELIPE STREET
HOUSTON
TX
77056
1
0
0
0
Marathon Oil Corporation Common Stock
2017-01-03
4
A
0
9853.604
0
A
26610.302
D
Marathon Oil Corporation Common Stock
7600
I
By Banister Resources Ltd.
Non-Retainer Annual Director Stock Unit Award granted under Marathon Oil Corporation 2016 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 238.849 shares previously not reported pursuant to Rule 16a-11.
These shares are held by Banister Resources Ltd., a limited partnership of which Banister Management Trust (the "Trust") is the general partner, and the reporting person and his spouse are the co-trustees of the Trust.
Kim Warnica, Attorney-in-Fact for Gaurdie E. Banister, Jr.
2017-01-04