Lirophora obliterata (W.H. Dall, 1902)
Pitar obscurate (FOSSIL) White, 1887 †
Periglypta obsoleta (FOSSIL) (G.P. Deshayes, 1857 †)
Dosinia occidentalis (FOSSIL) N.H. Ludbrook, 1978 †
Callista ogormani (FOSSIL) A.E.M. Cossmann, 1923 †
Liocyma okobikaensis (FOSSIL) Popova, 1994 †
Macrocallista olgensis (FOSSIL) Ilyina, 1963 †
Petricola olssoni F.R. Bernard, 1983
Megapitaria olssoni (FOSSIL) Marks 1951 †
Pitar omissa (H.A. Pilsbry & H.N. Lowe,1932)
Dosinia ongleyi (FOSSIL) J. Marwick, 1927 †
Marcia opima (J.F. Gmelin, 1791)
Marcia oppenoorthi (FOSSIL) C.H. Oostingh, 1935 †
Dosinia orbicularis (FOSSIL) A.E.R. Agassiz, 1845 †
Periglypta orbicularis (FOSSIL) (Hislop, 1859 †)
Dosinia orbiculata R.W. Dunker, 1877
Pitar orbiculata (FOSSIL) Berezovsky, 2010 †
Cyclorisma orbiculata (FOSSIL) L.W. Stephenson, 1952 †
Dosinia orbignyi R.W. Dunker, 1845
Cyclina orientalis (G.B. II Sowerby, 1852)
Flaventia orientalis (FOSSIL) J.H.F. Douvillé, 1916 †
Globivenus orientalis (L.R. Cox, 1930)
Lioconcha ornata (L.W. Dillwyn, 1817)
Chionopsis ornatissima (W.J. Broderip, 1835)
Pitar ornatus (FOSSIL) Devjatilova, 1981 †
Pleiorytis orthra (FOSSIL) W.P. Woodring, 1982 †
Pitar osmunda (T. Iredale, 1936)
Proxichione otiakensis (FOSSIL) P.A. Maxwell, 1978 †
Chionella ovalina (FOSSIL) (G.P. Deshayes, 1857 †)
Flaventia ovalis (FOSSIL) (J. de C. Sowerby) †
Timoclea ovata (T. Pennant, 1777)
Pleiorytis ovata (FOSSIL) T.A. Conrad, 1862 †
Marama ovata (FOSSIL) J. Marwick, 1927 †
Pitar ovatus (FOSSIL) (B. Rogers & B. Rogers,1837 †)
Bassina pachyphylla (I.H. Jonas, 1839)