- Oakleaf Crab
- Ocean Barracuda
- Ocean Bream
- Ocean Butterflyfish
- Ocean Catfish
- Ocean Chub
- Ocean Eel
- Ocean Jellyfish
- Ocean Manta Ray
- Ocean Perch
- Ocean Quahog
- Ocean Sand Dab
- Ocean Shark
- Oceanic Anglerfish
- Oceanic Bonito
- Oceanic Dolphin
- Oceanic Flying Fish
- Oceanic Pufferfish
- Oceanic Triggerfish
- Oceanic Whitetip Shark
- Octopus
- Ochre Sea Star
- Odontaspis (Sand Tiger Shark)
- Oilfish
- Oily Scad
- Olive Ridley Sea Turtle
- Olive Rockfish
- Onefin Skate
- One-Spot Snapper
- Opae Ula (Hawaiian Red Shrimp)
- Opah (Moonfish)
- Opaleye Fish
- Open-Sea Shrimp
- Orangemouth Corvina
- Orange Filefish
- Orange Garibaldi
- Orange Roughy
- Orange-Spotted Blenny
- Orange-Spotted Filefish
- Orange-Spotted Grouper
- Orange-Spotted Stingray
- Orange-Banded Surgeonfish
- Orbiculate Batfish
- Orca (Killer Whale)
- Ornate Butterflyfish
- Ornate Wrasse
- Ostracod (Seed Shrimp)
- Oyster
- Oyster Crab
- Oyster Drill Snail
- Oyster Toadfish
- Oyster Worm
- Oze Sandfish
