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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2021-03-31</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001221455</rptOwnerCik>
            <rptOwnerName>Ladhani Holli C.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O MARATHON OIL CORPORATION</rptOwnerStreet1>
            <rptOwnerStreet2>5555 SAN FELIPE</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>2021-03-31</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>10533.708</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>10533.708</value>
                </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 the 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>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Kim O. Warnica, Attorney-in-Fact for Holli C. Ladhani</signatureName>
        <signatureDate>2021-04-01</signatureDate>
    </ownerSignature>
</ownershipDocument>
[{"type": "4", "sequence": "1", "filename": "wf-form4_161730958457051.xml", "description": "FORM 4", "secsgml_size_bytes": 3516}]
X0306
4
2021-03-31
0
0000101778
MARATHON OIL CORP
MRO
0001221455
Ladhani Holli C.
C/O MARATHON OIL CORPORATION
5555 SAN FELIPE
HOUSTON
TX
77056
1
0
0
0
Common Stock
2021-03-31
4
A
0
10533.708
0
A
10533.708
D
Non-Retainer Annual Director Stock Unit Award granted under the 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.
Kim O. Warnica, Attorney-in-Fact for Holli C. Ladhani
2021-04-01