O
- Obsolete: faintly marked, scarcely evident
- Obtuse: blunt
- Occipital: pertaining to the posterior part of the skull
- Ocellus: an eye-like spot
- Ocular: pertaining to the eye
- Odontode: a dermal tooth
- Oesophagus: the gullet
- Opercle: the large bone which forms the upper posterior part of the operculum
- operculum: the bony flap that covers the gills
- Opercular: pertaining to the operculum
- Opercular spine: spine projecting from the operculum (view the fish figure)
- Orbit: the eye socket
- Origin: the most anterior point of a fin base
- Osseus: bony
- Ovate: egg-shaped
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