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

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2008-05-23</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000353230</issuerCik>
        <issuerName>OMNICARE INC</issuerName>
        <issuerTradingSymbol>OCR</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001418814</rptOwnerCik>
            <rptOwnerName>ValueAct Holdings, L.P.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>435 PACIFIC AVENUE, 4TH FLOOR</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>SAN FRANCISCO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94133</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001418812</rptOwnerCik>
            <rptOwnerName>VA Partners I, LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>435 PACIFIC AVENUE, 4TH FLOOR</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>SAN FRANCISCO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94133</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001375350</rptOwnerCik>
            <rptOwnerName>VA Partners III, LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>435 PACIFIC AVENUE, 4TH FLOOR</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>SAN FRANCISCO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94133</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001351069</rptOwnerCik>
            <rptOwnerName>ValueAct Capital Management, L.P.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>435 PACIFIC AVENUE, 4TH FLOOR</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>SAN FRANCISCO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94133</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001351073</rptOwnerCik>
            <rptOwnerName>ValueAct Capital Management, LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>435 PACIFIC AVENUE, 4TH FLOOR</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>SAN FRANCISCO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94133</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001418813</rptOwnerCik>
            <rptOwnerName>ValueAct Holdings GP, LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>435 PACIFIC AVENUE, 4TH FLOOR</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>SAN FRANCISCO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94133</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2008-05-23</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>400</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>400</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2008-05-23</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>5219</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                    <footnoteId id="F4"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>5619</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>10219330</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                    <footnoteId id="F5"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1563336</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                    <footnoteId id="F6"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Represents unrestricted shares awarded to Jeffrey W. Ubben as compensation for Mr. Ubben's service as an outside director of Omnicare, Inc. pursuant to the Company Plan.</footnote>
        <footnote id="F2">Under an agreement with ValueAct Capital, Jeffrey W. Ubben is deemed to hold the Common Stock for the benefit of ValueAct Capital Master Fund, L.P. &amp; indirectly for (i) VA Partners I, LLC as General Partner (&quot;GP&quot;) of ValueAct Capital Master Fund, L.P. (ii) ValueAct Capital Management, L.P. as the manager of ValueAct Capital Master Fund, L.P. (iii) ValueAct Capital Management, LLC as GP of ValueAct Capital Management, L.P. (iv) ValueAct Holdings, L.P. as the sole owner of the limited partnership interests of ValueAct Capital Management, L.P. &amp; the membership interests of ValueAct Capital Management, LLC &amp; as the majority owner of the membership interests of VA Partners I, LLC &amp; (v) ValueAct Holdings GP, LLC as GP of ValueAct Holdings, L.P.  The reporting persons disclaim beneficial ownership of the reported stock except to the extent of their pecuniary interest therein.</footnote>
        <footnote id="F3">Under an agreement with ValueAct Capital, Jeffrey W. Ubben is deemed to hold the Common Stock for the benefit of ValueAct Capital Master Fund III, L.P. &amp; indirectly for (i) VA Partners III, LLC as General Partner (&quot;GP&quot;) of ValueAct Capital Master Fund III, L.P (ii) ValueAct Capital Management, L.P. as the manager of ValueAct Capital Master Fund III, L.P. (iii) ValueAct Capital Management, LLC as GP of ValueAct Capital Management, L.P. (v) ValueAct Holdings, L.P. as the sole owner of the limited partnership interests of ValueAct Capital Management, L.P. &amp; the membership interests of ValueAct Capital Management, LLC &amp; as the majority owner of the membership interests of VA Partners III, LLC &amp; (vi) ValueAct Holdings GP, LLC as GP of ValueAct Holdings, L.P.  The reporting persons disclaim beneficial ownership of the reported stock except to the extent of their pecuniary interest therein.</footnote>
        <footnote id="F4">Represents restricted shares awarded to Jeffrey W. Ubben as compensation for Mr. Ubben's service as an outside director of Omnicare, Inc. pursuant to the Company Plan under which restrictions lapse three years after the date of grant.</footnote>
        <footnote id="F5">The reported stock is owned directly by ValueAct Capital Master Fund, L.P. and may be deemed to be beneficially owned by (i) VA Partners I, LLC as General Partner of ValueAct Capital Master Fund, L.P., (ii) ValueAct Capital Management, L.P. as the manager of of ValueAct Capital Master Fund, L.P., (iii) ValueAct Capital Management, LLC as General Partner of ValueAct Capital Management, L.P., (iv) ValueAct Holdings, L.P. as the sole owner of the limited partnership interests of ValueAct Capital Management, L.P. and the membership interests of ValueAct Capital Management, LLC and as the majority owner of the membership interests of VA Partners I, LLC and (v) ValueAct Holdings GP, LLC as General Partner of ValueAct Holdings, L.P.</footnote>
        <footnote id="F6">The reported stock is owned directly by ValueAct Capital Master Fund III, L.P. and may be deemed to be beneficially owned by (i) VA Partners III, LLC as General Partner of ValueAct Capital Master Fund III, L.P., (ii)  ValueAct Capital Management, L.P. as the manager of ValueAct Capital Master Fund III, L.P., (iii) ValueAct Capital Management, LLC as General Partner of ValueAct Capital
Management, L.P., (iv) ValueAct Holdings, L.P. as the sole owner of the limited partnership interests of ValueAct Capital Management, L.P. and the membership interests of ValueAct Capital Management, LLC and as the majority owner of the membership interests of VA Partners III, LLC and (v) ValueAct Holdings GP, LLC as General Partner of ValueAct Holdings, L.P.</footnote>
    </footnotes>

    <remarks>Joint Filer Information:

Name: ValueAct Capital Master Fund, L.P.
Address: 435 Pacific Avenue, 4th Floor, San Francisco, CA 94133
Designated Filer: ValueAct Holdings, L.P.
Issuer and Ticker: Omnicare, Inc. (OCR)
Date of Event Requiring Statement: 5/23/08

Name: ValueAct Capital Master Fund III, L.P.
Address: 435 Pacific Avenue, 4th Floor, San Francisco, CA 94133
Designated Filer: ValueAct Holdings, L.P.
Issuer and Ticker: Omnicare, Inc. (OCR)
Date of Event Requiring Statement: 5/23/08

Name: VA Partners I, LLC
Address: 435 Pacific Avenue, 4th Floor, San Francisco, CA 94133
Designated Filer: ValueAct Holdings, L.P.
Issuer and Ticker: Omnicare, Inc. (OCR)
Date of Event Requiring Statement: 5/23/08

Name: VA Partners III, LLC
Address: 435 Pacific Avenue, 4th Floor, San Francisco, CA 94133
Designated Filer: ValueAct Holdings, L.P.
Issuer and Ticker: Omnicare, Inc. (OCR)
Date of Event Requiring Statement: 5/23/08

Name: ValueAct Capital Management, L.P.
Address: 435 Pacific Avenue, 4th Floor, San Francisco, CA 94133
Designated Filer: ValueAct Holdings, L.P.
Issuer and Ticker: Omnicare, Inc. (OCR)
Date of Event Requiring Statement: 5/23/08

Name: ValueAct Capital Management, LLC
Address: 435 Pacific Avenue, 4th Floor, San Francisco, CA 94133
Designated Filer: ValueAct Holdings, L.P.
Issuer and Ticker: Omnicare, Inc. (OCR)
Date of Event Requiring Statement: 5/23/08

Name: ValueAct Holdings GP, LLC
Address: 435 Pacific Avenue, 4th Floor, San Francisco, CA 94133
Designated Filer: ValueAct Holdings, L.P.
Issuer and Ticker: Omnicare, Inc. (OCR)
Date of Event Requiring Statement: 5/23/08</remarks>

    <ownerSignature>
        <signatureName>VALUEACT HOLDINGS, L.P., By: VALUEACT HOLDINGS GP, LLC, its General Partner, By: /s/ George F. Hamel. Jr., Chief Operating Officer</signatureName>
        <signatureDate>2008-05-28</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>VALUEACT CAPITAL MASTER FUND, L.P., By: VA PARTNERS I, LLC, its General Partner, By:/s/ George F. Hamel. Jr., Chief Operating Officer</signatureName>
        <signatureDate>2008-05-28</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>VALUEACT CAPITAL MASTER FUND III, L.P., By:  VA PARTNERS III, LLC, its General Partner, By:/s/ George F. Hamel. Jr., Chief Operating Officer</signatureName>
        <signatureDate>2008-05-28</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>VA PARTNERS I, LLC, By:/s/ George F. Hamel. Jr., Chief Operating Officer</signatureName>
        <signatureDate>2008-05-28</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>VA PARTNERS III, LLC, By:/s/ George F. Hamel. Jr., Chief Operating Officer</signatureName>
        <signatureDate>2008-05-28</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>VALUEACT CAPITAL  MANAGEMENT, L.P., By:  VALUEACT CAPITAL MANAGEMENT, LLC, its General Partner, By:/s/ George F. Hamel. Jr., Chief Operating Officer</signatureName>
        <signatureDate>2008-05-28</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>VALUEACT CAPITAL MANAGEMENT, LLC, By:/s/ George F. Hamel. Jr., Chief Operating Officer</signatureName>
        <signatureDate>2008-05-28</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>VALUEACT HOLDINGS GP, LLC, By:/s/ George F. Hamel. Jr., Chief Operating Officer</signatureName>
        <signatureDate>2008-05-28</signatureDate>
    </ownerSignature>
</ownershipDocument>
