- Oblique
- having a slanting or sloping direction or position
- Oceanography
- study of the science of the world's ocean
- Ocelli
- non-imaging producing, small cup shaped structures at tentacle bases of jellies that function as light sensing organs
- Olfactory
- pertaining to the sense of smell
- Omnivore
- animal that feeds on both animal and plant matter
- Operculum
- hard flap serving as a cover for the the gill slits in fishes or the opening of the shell in some gastropods when the body is retracted into the shell
- Opportunistic feeder
- animal that feeds on whatever is available
- Oral
- of or pertaining to the mouth
- Oral arms
- structures around the mouth of jellies that aid in the capture and ingestion of food; may contain stinging cells and serve in sexual reproduction process
- Organic
- derived from living organisms
- Osmoregulator
- physiological mechanism used in an organism to regulate salt and fluid balance of the body to an acceptable level
- Oviparous
- egg-laying with young hatching outside female's body
- Ovoviviparity
- (now called aplacental viviparity). Eggs are incubated and hatched within the female's body, but without the formation of a placenta. Young are born live.
