- Longnose Batfish
Ogcocephalus corniger
- Key Brotula
Ogilbia cayorum
- Tidepool sculpin
Oligocottus maculosus
- Bank cusk-eel
Ophidion holbrookii
- Lingcod
Ophiodon elongatus
- Oyster toadfish
Opsanus tau
- ray-finned fishes
Actinopterygii - rays, sharks, and relatives
Chondrichthyes
- Cichlids
Cichlidae - Oreochromis
- Carp
Oreochromis esculentus
- 智利山鮰
Orestias ascotanensis
- Japanese killifish
Oryzias latipes
- Giant gourami
Osphronemus goramy
- Arawana
Osteoglossum bicirrhosum