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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2021-12-10</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001412350</issuerCik>
        <issuerName>PIMCO Income Opportunity Fund</issuerName>
        <issuerTradingSymbol>PKO</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001708751</rptOwnerCik>
            <rptOwnerName>TSU BRYAN</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O PIMCO</rptOwnerStreet1>
            <rptOwnerStreet2>650 NEWPORT CENTER DRIVE</rptOwnerStreet2>
            <rptOwnerCity>NEWPORT BEACH</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>92660</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>1</isOther>
            <otherText>SEE REMARKS</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>COMMON SHARES</value>
            </securityTitle>
            <transactionDate>
                <value>2021-12-10</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>D</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>1000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">The shares of the Issuer were disposed of in exchange for shares of PIMCO Dynamic Income Fund (&quot;PDI&quot;) in connection with the reorganization of the Issuer with and into PDI based on PDI's and the Issuer's respective net asset value per share at the time of the closing of such reorganization, which resulted in an exchange ratio of 0.949037.</footnote>
    </footnotes>

    <remarks>Pacific Investment Management Company LLC (&quot;PIMCO&quot;) is the investment adviser of the Issuer. The Reporting Person is a Portfolio Manager for this Issuer at PIMCO.</remarks>

    <ownerSignature>
        <signatureName>/s/ Kevin Van Gorder, Attorney-in-Fact for Bryan Tsu</signatureName>
        <signatureDate>2021-12-14</signatureDate>
    </ownerSignature>
</ownershipDocument>
