Science magazines
- O'ahu Creeper
- Oahu Tree Snails
- Ochlockonee Moccasinshell
- Okaloosa Darter
- Okeechobee Gourd
- Olive Ridley Sea Turtle
- Opuhe
- Orangefoot Pimpleback
- Orangenacre Mucket
- Orcutt's Spineflower
- Oregon Chub
- Oregon Silverspot Butterfly
- Osterhout Milk-vetch
- Otay Mesa Mint
- Otay Tarplant
- Ou
- Ouachita Rock-pocketbook
- Oval Pigtoe
- Ovate Clubshell
- Owens Pupfish