- †Oamaruia
- †Obex
- †Obexomia
- †Obornella
- Obtortio
- †Obtusella
- Ocenebra
- †Ochetochilus
- †Ochetoclava
- †Ocinebrellus
- †Ocinebrina
- †Odetta
- †Odoardia
- †Odontobasis
- †Odontofusus
- †Odontomaria
- †Odontopolys
- †Odontopurpura
- †Odontoturbo
- †Odostomella
- Odostomia
- †Odostomidea
- †Oehlertia
| - Oenopota
- †Offleya
- †Offlyeotrochus
- †Ogivia
- †Okinawavoluta
- †Olequahia
- †Oligoptycha
- †Oligoptyxis
- Oliva
- †Olivancillaria
- Olivella
- †Olivia
- †Olivula
- †Olssonella
- †Omalaxis
- Omalogyra
- †Omanimaria
- †Omogymna
- †Omospira
- †Omphalius
- †Omphalonema
- †Omphaloptycha
- †Omphalotrochus
| - Omphalotropis
- †Oncochilus
- †Ongleya
- †Oniscidia
- †Onkospira
- †Onoba
- Oocorys
- †Ooliticia
- †Oonia
- †Oostrombus
- †Ootomaria
- †Ootomella
- Opalia
- †Opaliopsis
- †Opella
- †Ophileta
- †Ophiletina
- Ophiodermella
- †Opimilda
- †Opisthonema
- †Optoturris
- Orbitestella
- †Orecopia
| - †Orectospira
- †Orinella
- †Ormastralium
- †Ornamentaria
- †Ornatoptygmatis
- †Ornatospira
- †Ornopsis
- †Orospira
- †Orthaulax
- †Orthochetus
- †Orthochilus
- †Orthonema
- †Orthonychia
- †Orthoptyxis
- †Orthostomia
- †Orthosurcula
- †Osilinus
- †Ossiania
- †Ostioma
- †Otatara
- †Otollonia
- †Otomphalus
- †Otopleura
| - †Otostoma
- †Ottoino
- Otukaia
- †Ovactaeonina
- †Ovatipsa
- †Ovilia
- †Ovinotis
- †Ovirissoa
- †Ovulactaeon
- †Ovulopsis
- †Owenella
- †Oxyacrum
- †Oxycypraea
- †Oxygyrus
- †Oxymeris
- †Oxyspira
- †Oxystele
- †Ozarkina
- †Ozarkispira
- †Ozodochilus
| |
