- obesulus May, 1919 = Cystiscus obesulus (family Cystiscidae – Cystiscinae)
- obesus H. Adams, 1866 = Amphithalamus obesus (family Anabathridae)
- oblata Laseron, 1956 = Pisinna oblata (family Anabathridae)
- obliqua Lamarck, 1819 = Ennucula obliqua (family Nuculidae)
- obliqua May, 1915 = Inella obliqua (family Triphoridae – Triphorinae)
- obliquissima Tate, 1887 = Propecuna obliquissima (family Condylocardiidae – Cuninae)
- obliquus Hedley, 1902 = Cyclochlamys favus (family Propeamussiidae)
- obliquus Reeve, 1843 = Glycymeris striatularis (family Glycymerididae)
- oblivirgulata Hubendick, 1943 = Siphonaria funiculata (family Siphonariidae)
- oblonga Sowerby, 1830 = Amalda oblonga (family Olividae – Ancillariinae)
- obnixa Iredale, 1936 = Cancellaria undulata (family Cancellariidae – Cancellariinae)
- obscura Hutton, 1873 = Waimatea obscura (family Volutomitridae)
- obscura Laseron, 1954 = Brookula angeli (family Turbinidae – Skeneinae)
- obturamentum Hedley, 1893 = Barnea obturamentum (family Pholadidae – Pholadinae)
- obtusa Laseron, 1955 = Curveulima obtusa (family Eulimidae)
- obtusa Petterd, 1886 = Amalda obtusa (family Olividae – Ancillariinae)
- obtusa Crosse & Fischer, 1864 = Paphies cuneata (family Mesodesmatidae – Mesodesmatinae)
- obtusa Guiler, 1951 (non Dillwyn) (in part) = Austrocochlea constricta (family Trochidae – Trochinae)
- obtusa May, 1923 (in part) = Austrocochlea brevis (family Trochidae – Trochinae)
- obtusus of authors (non Dillwyn) = Austrocochlea porcata (family Trochidae – Trochinae)
- occasiuncula Iredale, 1932 = Bankia australis (family Teredinidae – Bankiinae)
- occidentalis Iredale, 1935 = Notocypraea piperita (family Cypraeidae – Pustulariinae)
- occiduus Cotton & Godfrey, 1934 = Clanculus undatus (family Trochidae – Trochinae)
- occiduus Verco, 1911 = Gadila occiduus (family Gadilidae)
- occultidens May, 1915 = Odostomia occultidens (family Pyramidellidae – Odostomiinae)
- ocellatus Gould, 1862 = Phasianotrochus eximius (family Trochidae – Trochinae)
- ochracea Angas, 1871 = Alaginella ochracea (family Marginellidae – Marginellinae)
- ochroleuca Henn & Brazier, 1894 = Onoba australiae (family Rissoidae – Rissoinae)
- ochroleucus Philippi, 1849 = Clanculus albanyensis (family Trochidae – Trochinae)
- octona Tate, 1891 = Leiopyrga octona (family Trochidae – Umboniinae)
- octopleuron Verco, 1911 = Dentalium octopleuron (family Dentaliidae)
- odontis Wood, 1828 = Chlorodiloma odontis (family Trochidae – Trochinae)
- oleacea Hedley & Petterd, 1906 = Archiminolia oleacea (family Solariellidae)
- oligomphala Pilsbry, 1895 = Tricolia variabilis (family Phasianellidae – Tricoliinae)
- oligostira Tate, 1891 = Penion mandarinus (family Buccinidae – Siphonaliinae)
- olivacea Frauenfeld, 1867 = Pisinna olivacea (family Anabathridae)
- olivella Reeve, 1865 = Mesoginella olivella (family Marginellidae – Marginellinae)
- omicron Crosse & Fischer, 1864 = Amblychilepas omicron (family Fissurellidae – Fissurellinae)
- onuphria Iredale, 1936 = Soletellina alba (family Psammobiidae)
- ophione Gray, 1849 = Lamellaria ophione (family Velutinidae – Lamellariinae)
- optata Gould, 1860 = Nassarius nigellus (family Nassariidae – Nassariinae)
- opulenta Hedley, 1907 = Glyptozaria opulenta (family Cerithiidae – Cerithiinae)
- orbicularis Angas, 1879 = Tucetona flabellata (family Glycymerididae)
- orbita Gmelin, 1791 = Dicathais orbita (family Muricidae – Rapaninae)
- orientalis A. Adams & Reeve, 1850 = Limaria orientalis (family Limidae)
- ornata Hedley, 1922 = Filodrillia ornata (family Conidae – Clathurellinae)
- ornata Angas, 1864 = Austraeolis ornata (family Facelinidae – Favorininae)
- ornata May, 1908 = Scissurella ornata (family Scissurellidae – Scissurellinae)
- ornata Tenison Woods, 1876 = Astralium aureum (family Turbinidae – Turbininae)
- ornatum Tokioka & Baba, 1964 = Sagaminopteron ornatum (family Gastropteridae)
- ornatus Férussac, 1821 = Ophicardelus ornatus (family Ellobiidae – Ellobiinae)
- ornatus Tenison Woods, 1876 = Phasianotrochus apicinus (family Trochidae – Trochinae)
- orospatia Laseron, 1954 = Brookula angeli (family Turbinidae – Skeneinae)
- orthopleura Tate, 1898 = Melanella orthopleura (family Eulimidae)
- orukta Maughan, 1900 = Rhyssoplax orukta (family Chitonidae – Chitoninae)
- oryzata Meuschen, 1787 = Argonauta nodosus (family Argonautidae)
- ostanes Iredale, 1954 = Sepia novaehollandiae (family Sepiidae)
- ostenta Burn, 1958 = Doto ostenta (family Dotidae)
- otagoensis Powell, 1954 = Cabestana tabulata (family Ranellidae – Cymatiinae)
- ovata Hedley, 1906 = Mysella ovata (family Galeommatidae)
- ovata Lamarck, 1819 = Vulsella spongiarum (family Malleidae)
- ovatum Tate, 1886 = Lepton ovatum (family Galeommatidae)
- ovulaeformis Tate & May, 1900 = Cystiscus bucca (family Cystiscidae – Cystiscinae)
- ovulum Sowerby, 1846 = Ovaginella ovulum (family Marginellidae – Marginellinae)
- oxyacris Tate, 1897 = Gazameda tasmanica (family Turritellidae – Turritellinae)