- oaxacae
- oblonga
- obtuse
- occidentalis
- occipital condyle
- occiput
- ocellata
- ocellus
- oculifera
- odoratus
- oldhamii
- olfaction
- olfactory nerves
- olivacea
- omnivorous
- opisthocoelous
- opisthotic
- orbicularis
- orbicularis-type
- orbit
- orbital rim
- orbito-nasal foramen
- orbitomandibular
- orientalis
- ornata
- osceola
- osmotrophs
- ossify
- osteology
- otic
- otic capsule
- otic notch
- otolithic
- ouachitensis
- ovarian follicles
- oviduct
- oviductal lumen
- oviposit