| Oarfishes |
| Object Oriented |
| objective synonym |
| obligate lecithotrophic live bearer |
| obligate mutualism |
| obligatory |
| observer |
| obsolescent |
| obsolete |
| obtuse |
| Occam's razor |
| Occidental College, California |
| occipital |
| occipital centrum |
| occipital region |
| occiput |
| occlusal |
| occlusible |
| ocean ecosystems |
| Oceania |
| oceanic |
| oceanic benthic ecosystems |
| oceanic seamount |
| oceanodromous |
| oceanographic survey |
| oceanography |
| ocellate |
| ocellus |
| octopus |
| ocular |
| odd-year run |
| oedema |
| oesophagus |
| off-site |
| offal |
| offshore |
| offshore fishery |
| offshore plain |
| offshore waters |
| Ohio State University Museum of Zoology |
| oil globule |
| oily fish |
| Okinawa Regional Research Center |
| Oklahoma Museum of Natural History |
| Oklahoma State University at Stillwater |
| Old Dominion University, Virginia |
| Oldwife |
| olfactory |
| olfactory bulb |
| olfactory organ |
| olfactory pit |
| oligo- |
| Oligocene |
| Oligochaeta |
| oligohaline |
| oligomictic |
| oligophagous |
| oligosaline |
| oligothermic |
| oligotrophic |
| olim |
| Oman moray |
| Oman Natural History Museum, Muscat |
| omega-3-fatty acid |
| omni- |
| omnivore |
| on-line database |
| on-site |
| ongrowing |
| onomatography |
| onshore waters |
| ontogenesis |
| ontogenetic characters |
| ontogeny |
| oo |
| oocyte |
| oogonia |
| oophagy |
| ooze |
| Opahs |
| open access fishery |
| open access resource |
| operating system |
| opercle |
| opercular |
| opercular canal |
| opercular opening |
| opercular pumping |
| operculum |
| opistho- |
| opisthocoelous |
| opisthonephros |
| opisthonephrus |
| opisthure |
| opportunistic feeding |
| opportunity cost |
| optic cup |
| optic nerve foramen |
| optic pedicel |
| optic primordium |
| optic tectum |
| optic vesicles |
| Optical Character Recognition |
| optimum age |
| optimum length |
| optimum yield |
| oral |
| oral brooder |
| oral cavity |
| oral disc |
| oral disk |
| oral fimbriae |
| oral gestation |
| oral incubation |
| oral plate |
| oral valve |
| orange meat |
| orbicular |
| orbicular stage |
| Orbicular velvetfishes |
| orbit |
| orbital |
| orbital bones |
| orbital diameter |
| orbital region |
| Ordivician |
| Oreos |
| oreosoma |
| organic |
| organism |
| organogenesis |
| organoleptic |
| Oriental |
| oriental cure |
| origin |
| original description |
| original designation |
| original publication |
| orobranchial chamber |
| oronasal curtain |
| oronasal groove |
| ortho- |
| orthographic variants |
| orthography |
| osmoregulation |
| OSMOSE |
| osseus |
| ossicle |
| ossification |
| ossify |
| ostariophysan |
| Ostariophysi |
| Osteichthyes |
| osteocranium |
| osteoderm |
| osteology |
| ostraciform |
| ostracin |
| ostracitoxin |
| ostracods |
| otic |
| otic capsule |
| otic region |
| Otic vesicle |
| otocyst |
| otolith |
| otter |
| otter board |
| otter trawl |
| ouananiche |
| outgroup |
| outlier male |
| output controls |
| ova |
| oval |
| ovarian tunic |
| ovary |
| Ovas secas |
| ovate |
| over stock |
| over-feeding |
| overfished |
| overfishing |
| overlapping dentition |
| overturn |
| oviduct |
| oviparity |
| oviparous |
| oviphagous |
| oviposit |
| ovipositor |
| ovo-testes |
| ovoid |
| ovophile |
| ovoviviparity |
| ovoviviparous |
| ovum |
| oxbow |
| oxbow lake |
| oxycercal |
| oxygen consumption |
| oxyluciferin |