Octobranchus floriceps Kingston & Mackie, 1980 – synonym of Octobranchus floriceps
Odontosyllis ctenostoma Claparède, 1868 – synonym of Odontosyllis ctenosoma
Odontosyllis fulgurans McIntosh, 1908 – synonym of Odontosyllis fulgurans
Odontosyllis gibba Fauvel, 1923 – synonym of Odontosyllis gibba
Odontosyllis gibba var. robertianae – synonym of Odontosyllis gibba
Odontosyllis gibba var. robertianae McIntosh, 1908 – synonym of Odontosyllis gibba
Oniscosoma arcticum – synonym of Spinther arcticus
Oniscosoma arcticum M. Sars, 1851 – synonym of Spinther arcticus
Onuphis britannica – synonym of Nothria conchylega
Onuphis britannica McIntosh, 1903 – synonym of Nothria conchylega
Onuphis conchylega – synonym of Nothria conchylega
Onuphis conchylega Sars, 1835 – synonym of Nothria conchylega
Onuphis fiordica – synonym of Paradiopatra fiordica
Onuphis fiordica Hartmann-Schröder, 1985 – synonym of Paradiopatra fiordica
Onuphis quadricuspis – synonym of Paradiopatra quadricuspis
Onuphis quadricuspis M. Sars, 1872 – synonym of Paradiopatra quadricuspis
Ophelia borealis – synonym of Ophelia limacina
Ophelia borealis Tebble, 1952 – synonym of Ophelia limacina
Ophelia cluthensis – synonym of Ophelia rathkei
Ophelia cluthensis McGuire, 1935 – synonym of Ophelia rathkei
Ophelia limacina Fauvel, 1927 – synonym of Ophelia limacina
Ophelia mamillata – synonym of Travisia forbesii
Ophelia mamillata Oersted, 1843 – synonym of Travisia forbesii
Ophelia rathkei McIntosh, 1908 – synonym of Ophelia rathkei
Ophelia remanei – synonym of Ophelia rathkei
Ophelia remanei Augener, 1940 – synonym of Ophelia rathkei
Ophelina abranchiata Støp-Bowitz, 1948 – synonym of Ophelina abranchiata
Ophelina acuminata Oersted, 1843 – synonym of Ophelina acuminata
Ophelina cylindricaudata Støp-Bowitz, 1945 – synonym of Ophelina cylindricaudata
Ophelina minima Hartmann-Schröder, 1974 – synonym of Ophelina minima
Ophelina modesta Støp-Bowitz, 1958 – synonym of Ophelina modesta
Ophelina norvegica Støp-Bowitz, 1945 – synonym of Ophelina norvegica
Ophiodromus flexuosis McIntosh, 1908 – synonym of Ophiodromus flexuosus
Ophiodromus roseus – synonym of Gyptis rosea
Ophiodromus roseus Malm, 1874 – synonym of Gyptis rosea
Ophiodromus vittatus – synonym of Ophiodromus flexuosus
Ophiodromus vittatus Tauber, 1879 – synonym of Ophiodromus flexuosus
Ophryotrocha (?) isochaeta Eliason, 1962 – synonym of Parophryotrocha isochaeta
Ophryotrocha dubia Hartmann-Schröder, 1974 – synonym of Ophryotrocha dubia
Ophryotrocha gerlachi Hartmann-Schröder, 1974 – synonym of Ophryotrocha gerlachi
Ophryotrocha geryonicola Hartmann-Schröder, 1971 – synonym of Ophryotrocha geryonicola
Ophryotrocha gracilis Huth, 1934 – synonym of Ophryotrocha gracilis
Ophryotrocha hartmanni Huth, 1933 – synonym of Ophryotrocha hartmanni
Ophryotrocha longidentata Josefson, 1975 – synonym of Ophryotrocha longidentata
Ophryotrocha maculata Åkesson, 1973 – synonym of Ophryotrocha maculata
Ophryotrocha minuta – synonym of Arenotrocha minuta
Ophryotrocha minuta Levi, 1954 – synonym of Arenotrocha minuta
Ophryotrocha puerilis Claparède & Mecznikow, 1869 – synonym of Ophryotrocha puerilis
Ophryotrocha socialis Ockelmann & Åkesson, 1990 – synonym of Ophryotrocha socialis
Opisthodonta pterochaeta Southern, 1914 – synonym of Opisthodonta pterochaeta
Orbinia armandi Hartmann-Schröder, 1974 – synonym of Orbinia armandi
Orbinia kupfferi Hartmann-Schröder, 1971 – synonym of Orbinia kupfferi
Orbinia latreillii Hartmann-Schröder, 1971 – synonym of Orbinia latreillii
Orbinia norvegica Hartmann-Schröder, 1971 – synonym of Orbinia norvegica
Orbinia sertulata Hartmann-Schröder, 1971 – synonym of Orbinia sertulata
Ougia subaequalis Wolf, 1986 – synonym of Ougia subaequalis
Owenia assimilis – synonym of Owenia fusiformis
Owenia assimilis Michaelsen, 1896 – synonym of Owenia fusiformis
Owenia filiformis – synonym of Owenia fusiformis
Owenia filiformis Michaelsen, 1896 – synonym of Owenia fusiformis
Owenia fusiformis Delle Chiaje, 1842 – synonym of Owenia fusiformis