- oblique
- oblong
- obtuse
- occipital
- occiput
- ocean
- oceanic
- Oceanography
- ocellus
- octagonal
- ocular
- offshore
- olfactory
- Oligocene
- oligohaline
- omnivorous
- ontogeny
- oocyte
- Oogenesis
- opalescent
- opercle
- opportunism
- oral
- orbit
- order
- organ
- orifice
- ornament
- oro-buccal
- osmoregulatory
- ossified
- osteology
- ostracods
- ostrum
- otic
- otolith
- ovarian
- ovary
- ovate
- oviparous
- oviposit
- ovulation
- ovum
- oxygen
- oxygen minimum layer
- oyster