- occipital
- oceanic
- Ocelli
- Octant
- ocular
- Ocular lappet
- Odontophore
- oesophagus
- oikoplasts
- Olfactory organ
- Oostegites
- oozooid
- opercle
- opercular
- Operculum
- Oral
- Oral arms
- Oral basket
- Oral bulge
- Oral cavity
- Oral cone
- Oral disc
- Oral process
- Oral spines
- Oral teeth
- Oral tentacle
- Orbital pore
- Orbital sinus
- organ
- organ of Bellonci
- ornament
- osphradium
- Ostium
- otolith
- Otoporpae
- Outer hump
- outer margin
- ovary
- ovum