O
- Obovaria retusa — Golf stick pearly mussel, Ring pink
- Obovaria rotulata — Round ebonyshell
- Ocadia sinensis — Chinese Stripe-necked Turtle
- Occirhenea georgiana
- Oceanites maorianus — New Zealand Storm-petrel
- Oceanodroma homochroa — Ashy Storm-petrel
- Oceanodroma macrodactyla — Guadalupe Storm-petrel
- Ochotona argentata — Helan Shan Pika, Silver Pika
- Ochotona koslowi — Koslov's Pika, Kozlov's Pika
- Odontophorus strophium — Gorgeted Wood-quail
- Odontophrynus moratoi
- Oecanthus laricis — Laricis Tree Cricket
- Oecomys cleberi — Cleber's Arboreal Rice Rat
- Oedipina altura
- Oedipina gephyra
- Oedipina gracilis
- Oedipina grandis
- Oedipina maritima
- Oedipina paucidentata
- Oedipina poelzi
- Oedipina pseudouniformis
- Oedipina stenopodia
- Ogasawarana chichijimana
- Ogasawarana habei
- Ogasawarana metamorpha
- Ogasawarana rex
- Ogasawarana yoshiwarana
- Ognorhynchus icterotis — Yellow-eared Parrot
- Okapia Johnstoni
- Oligoaeschna kunigamiensis
- Oligoryzomys victus — St. Vincent Pygmy Rice Rat
- Omphalotropis hieroglyphica
- Oncorhynchus apache — Apache trout
- Oncorhynchus formosanus
- Oncorhynchus gilae — Gila trout
- Oncorhynchus ishikawai — Satsukimasu salmon
- Onychogalea fraenata — Bridled Nail-tailed Wallaby, Bridled Nail-tail Wallaby
- Oreochromis jipe — Jipe tilapia
- Oreochromis karomo — Karomo
- Oreochromis karongae
- Oreochromis lidole
- Oreochromis pangani
- Oreochromis squamipinnis
- Oreochromis variabilis
- Oreocnemis phoenix
- Oreolalax chuanbeiensis
- Oreolalax liangbeiensis
- Oreolalax omeimontis
- Oreolalax pingii
- Oreolalax puxiongensis
- Oreomystis bairdi — 'Akikiki
- Oreomystis mana — Hawai'i Creeper
- Oreonax flavicauda — Yellow-tailed Woolly Monkey
- Oreophasis derbianus — Horned Guan
- Oreophryne monticola
- Oriolus isabellae — Isabela Oriole
- Orlitia borneensis — Bornean River Turtle, Malaysian Giant Turtle
- Ornithoptera alexandrae — Queen Alexandra's Birdwing
- Ornithoptera croesus
- Ornithoptera meridionalis
- Orthochromis kasuluensis
- Orangia maituatensis
- Orangia sporadica
- Orconectes saxatilis — Kiamichi Crayfish
- Orconectes shoupi — Nashville Crayfish
- Oreailurus jacobita — Andean Cat, Andean Mountain Cat
- Oreochromis amphimelas
- Oreochromis chungruruensis
- Oreochromis esculentus — Singidia tilapia
- Oreochromis hunteri — Lake Chala tilapia
- Oreochromis jipe — Jipe tilapia
- Oreochromis karomo — Karomo
- Oreochromis karongae
- Oreochromis lidole
- Oreochromis pangani
- Oreochromis squamipinnis
- Oreochromis variabilis
- Oreocnemis phoenix
- Oreolalax chuanbeiensis
- Oreolalax liangbeiensis
- Oreolalax omeimontis
- Oreolalax pingii
- Oreolalax puxiongensis
- Oreomystis bairdi — 'Akikiki
- Oreomystis mana — Hawai'i Creeper
- Oreonax flavicauda — Yellow-tailed Woolly Monkey
- Oreophasis derbianus — Horned Guan
- Oreophryne monticola
- Oriolus isabellae — Isabela Oriole
- Orlitia borneensis — Bornean River Turtle, Malaysian Giant Turtle
- Ornithoptera alexandrae — Queen Alexandra's Birdwing
- Ornithoptera croesus
- Ornithoptera meridionalis
- Orthochromis kasuluensis
- Orthochromis mazimeroensis
- Orthochromis mosoensis
- Orthochromis rubrolabialis
- Orthochromis uvinzae
- Orthogeomys cuniculus — Oaxacan Pocket Gopher
- Oryx leucoryx — Arabian Oryx, White Oryx
- Oryzias orthognathus — Buntingi, Sharp-jawed buntingi
- Oryzomys gorgasi — Gorgas's Rice Rat
- Osornophryne antisana
- Osornophryne guacamayo — Guacamayo Plump Toad
- Osornophryne percrassa — Herveo Plump Toad
- Osornophryne talipes — Cannatella's Plump Toad
- Ospatulus palaemophagus
- Ospatulus truncatus — Bitungu
- Osteopilus crucialis — Jamaican Snoring Frog
- Osteopilus marianae — Yellow Bromeliad Frog
- Osteopilus pulchrilineatus — Hispaniolan Yellow Treefrog
- Osteopilus vastus — Hispaniolan Giant Treefrog
- Osteopilus wilderi — Green Bromeliad Frog
- Otomops wroughtoni — Wroughton's Free-tailed Bat
- Otomys barbouri — Barbour's Vlei Rat
- Otomys jacksoni — Mount Elgon Vlei Rat
- Otomys uzungwensis — Uzungwe Vlei Rat
- Otus alfredi — Flores Scops-owl
- Otus beccarii — Biak Scops-owl
- Otus capnodes — Anjouan Scops-owl
- Otus insularis — Seychelles Scops-owl
- Otus ireneae — Sokoke Scops-owl
- Otus moheliensis — Moheli Scops-owl
- Otus pauliani — Grand Comoro Scops-owl
- Otus siaoensis — Siau Scops-owl
- Otus thilohoffmanni — Serendib Scops-owl
- Ovis canadensis sierrae - Sierra Nevada Bighorn Sheep
- Oxychaeta dicksoni — Dickson's Copper
- Oxylapia polli
- Oxyloma kanabense — Kanab Amber Snail
- Oxyura leucocephala — White-headed Duck
- Oziothelphusa dakuna
- Oziothelphusa gallicola
- Oziothelphusa intuta
- Oziothelphusa kodagoda
- Oziothelphusa populosa
Read more about this topic: IUCN Red List Endangered Animal Species