- Oncostoma olivaceum
- One-colored Becard
- One-wattled Cassowary
- Oneillornis
- Oneillornis lunulatus
- Oneillornis salvini
- Onkaparinga Gorge, Onkaparinga Wetlands(South Australia near Adelaide)
- Onkaparinga Gorge and Onkaparinga Wetlands
- Ontario
- Onychognathus
- Onychognathus albirostris
- Onychognathus blythii
- Onychognathus frater
- Onychognathus fulgidus
- Onychognathus morio
- Onychognathus nabouroup
- Onychognathus neumanni
- Onychognathus salvadorii
- Onychognathus tenuirostris
- Onychognathus tristramii
- Onychognathus walleri
- Onychoprion
- Onychoprion aleuticus
- Onychoprion anaethetus
- Onychoprion fuscatus
- Onychoprion lunata
- Onychoprion lunatus
- Onychorhynchus
- Onychorhynchus coronatus
- Onychorhynchus coronatus castelnaui
- Onychorhynchus coronatus swainsoni
- Onychorhynchus mexicanus
- Onychorhynchus occidentalis
- Onychorhynchus swainsoni
- Onychostruthus
- Onychostruthus taczanowskii
- Opal-crowned Manakin
- Opal-crowned Tanager
- Opal-rumped Tanager
- Openbill
- Openbills
- Ophrydornis
- Ophrydornis albogularis
- Ophrysia superciliosa
- Opisthocomidae
- Opisthocomus
- Opisthocomus hoazin
- Opisthoprora
- Opisthoprora euryptera
- Oporornis
- Oporornis agilis
- Oporornis formosus
- Oporornis philadelphia
- Oporornis tolmiei
- Optical Instruments for the Observation of Nature
- Ora Nature Reserve
- Ora Nature Reserve Norway
- Orange-backed Troupial
- Orange-backed Woodpecker
- Orange-banded Flycatcher
- Orange-banded Thrush
- Orange-barred Leaf Warbler
- Orange-bellied Antwren
- Orange-bellied Chloropsis
- Orange-bellied Euphonia
- Orange-bellied Flowerpecker
- Orange-bellied Fruit-Dove
- Orange-bellied Fruit Dove
- Orange-bellied Leafbird
- Orange-bellied Manakin
- Orange-bellied Parrot
- Orange-bellied Trogon
- Orange-billed Babbler
- Orange-billed Jungle Myna
- Orange-billed Lorikeet
- Orange-billed Nightingale-Thrush
- Orange-billed Nightingale Thrush
- Orange-billed Scimitar-Babbler
- Orange-billed Scimitar Babbler
- Orange-billed Sparrow
- Orange-breasted Bunting
- Orange-breasted Bush-Shrike
- Orange-breasted Bush-shrike
- Orange-breasted Bushshrike
- Orange-breasted Chat
- Orange-breasted Falcon
- Orange-breasted Fig-Parrot
- Orange-breasted Fig Parrot
- Orange-breasted Fruiteater
- Orange-breasted Green-Pigeon
- Orange-breasted Green Pigeon
- Orange-breasted Honeyeater
- Orange-breasted Laughingthrush
- Orange-breasted Myzomela
- Orange-breasted Pigeon
- Orange-breasted Rock-jumper
- Orange-breasted Rockjumper
- Orange-breasted Sunbird
- Orange-breasted Thornbird
- Orange-breasted Trogon
- Orange-breasted Waxbill
- Orange-browed Hemispingus
- Orange-browed Tanager
- Orange-cheeked Honeyeater
- Orange-cheeked Parrot
- Orange-cheeked Waxbill
- Orange-chinned Parakeet
- Orange-collared Manakin
- Orange-crested Bowerbird
- Orange-crested Flycatcher
- Orange-crested Manakin
- Orange-crowned Euphonia
- Orange-crowned Fairywren
- Orange-crowned Manakin
- Orange-crowned Oriole
- Orange-crowned Warbler
- Orange-eared Parrotbill
- Orange-eared Tanager
- Orange-eyed Flatbill
- Orange-eyed Flycatcher
- Orange-eyed Thornbird
- Orange-faced Grackle
- Orange-faced Myna
- Orange-flanked Bush Robin
- Orange-footed Scrubfowl
- Orange-footed megapode
- Orange-fronted Barbet
- Orange-fronted Chat
- Orange-fronted Fruit-Dove
- Orange-fronted Fruit Dove
- Orange-fronted Hanging-Parrot
- Orange-fronted Hanging Parrot
- Orange-fronted Nun
- Orange-fronted Parakeet
- Orange-fronted Plushcrown
- Orange-fronted Softtail
- Orange-fronted Yellow-Finch
- Orange-fronted Yellow-finch
- Orange-fronted Yellow Finch
- Orange-headed Ground-Thrush
- Orange-headed Ground Thrush
- Orange-headed Parrot
- Orange-headed Tanager
- Orange-headed Thrush
- Orange-necked Partridge
- Orange-sided Thrush
- Orange-spotted Bulbul
- Orange-templed Babbler
- Orange-throated Longclaw
- Orange-throated Sunangel
- Orange-throated Tanager
- Orange-tufted Spiderhunter
- Orange-tufted Sunbird
- Orange-winged Amazon
- Orange-winged Parrot
- Orange-winged Pytilia
- Orange-winged Sittella
- Orange Bishop
- Orange Bullfinch
- Orange Chat
- Orange Dove
- Orange Fruit Dove
- Orange Ground-Thrush
- Orange Ground Thrush
- Orange Minivet
- Orange Oriole
- Orange Parrotbill
- Orange River Francolin
- Orange River White-eye
- Orange Suthora
- Orange Weaver
- Orange chinned Parakeet
- Orangequit
- Orava Reservoir
- Orava Reservoir Slovakia
- Orbetello-Burano Reserves
- Orbetello-Burano Reserves Italy
- Orchard Oriole
- Orchesticus
- Orchesticus abeillei
- Ordesa National Park and adjacent Central Pyrenees Spain
- Ordesa National Park and adjacent central Pyrenees
- Oregon
- Oregon Junco
- Oreocharis
- Oreocharis arfaki
- Oreocharis stictoptera
- Oreoica
- Oreoica gutturalis
- Oreoicidae
- Oreolais
- Oreolais pulcher
- Oreolais ruwenzorii
- Oreomanes
- Oreomanes fraseri
- Oreomystis
- Oreomystis bairdi
- Oreomystis mana
- Oreonympha
- Oreonympha nobilis
- Oreophasis
- Oreophasis derbianus
- Oreophilais
- Oreophilais robertsi
- Oreopholus
- Oreopholus ruficollis
- Oreopholus ruficollis ruficollis
- Oreophylax
- Oreophylax moreirae
- Oreopsar
- Oreopsar bolivianus
- Oreopsittacus
- Oreopsittacus arfaki
- Oreopyra calolaema
- Oreornis
- Oreornis chrysogenys
- Oreortyx
- Oreortyx pictus
- Oreoscoptes
- Oreoscoptes montanus
- Oreoscopus
- Oreoscopus gutturalis
- Oreostruthus
- Oreostruthus fuliginosus
- Oreothlypis
- Oreothlypis celata
- Oreothlypis celata orestera
- Oreothlypis crissalis
- Oreothlypis gutturalis
- Oreothlypis luciae
- Oreothlypis peregrina
- Oreothlypis ruficapilla
- Oreothlypis superciliosa
- Oreothlypis virginiae
- Oreothraupis
- Oreothraupis arremonops
- Oreotrochilus
- Oreotrochilus adela
- Oreotrochilus chimborazo
- Oreotrochilus cyanolaemus
- Oreotrochilus estella
- Oreotrochilus leucopleurus
- Oreotrochilus melanogaster
- Oreotrochilus stolzmanni
- Oressochen
- Oressochen jubatus
- Oressochen melanopterus
- Orgambideska
- Orgambideska France
- Organbideska
- Oriental Bay-Owl
- Oriental Bay Owl
- Oriental Bullfinch
- Oriental Crow
- Oriental Cuckoo
- Oriental Darter
- Oriental Dollarbird
- Oriental Dwarf Kingfisher
- Oriental Eagle-Owl
- Oriental Eagle Owl
- Oriental Great Reed-Warbler
- Oriental Great Reed Warbler
- Oriental Greenfinch
- Oriental Hobby
- Oriental Honey-Buzzard
- Oriental Honey-buzzard
- Oriental Honey Buzzard
- Oriental Magpie
- Oriental Magpie-Robin
- Oriental Magpie Robin
- Oriental Paradise Flycatcher
- Oriental Pied-Hornbill
- Oriental Pied Hornbill
- Oriental Pipit
- Oriental Plover
- Oriental Pratincole
- Oriental Reed-Warbler
- Oriental Reed Warbler
- Oriental Scops-Owl
- Oriental Scops Owl
- Oriental Skylark
- Oriental Stork
- Oriental Turtle-Dove
- Oriental Turtle Dove
- Oriental White-backed Vulture
- Oriental White-eye
- Oriental White Ibis
- Oriental White Stork
- Oriente Warbler
- Origma
- Origma solitaria
- Orinocan Saltator
- Orinoco Goose
- Orinoco Piculet
- Orinoco Saltator
- Orinoco Softtail
- Oriole Blackbird
- Oriole Cuckoo-shrike
- Oriole Cuckooshrike
- Oriole Finch
- Oriole Warbler
- Oriole Whistler
- Oriolia
- Oriolia bernieri
- Oriolidae
- Oriolus
- Oriolus Chinensis
- Oriolus albiloris
- Oriolus auratus
- Oriolus bouroensis
- Oriolus brachyrhynchus
- Oriolus brachyrynchus
- Oriolus chinensis
- Oriolus chinensis andamanensis
- Oriolus chlorocephalus
- Oriolus crassirostris
- Oriolus cruentus
- Oriolus decipiens
- Oriolus flavocinctus
- Oriolus forsteni
- Oriolus hosii
- Oriolus isabellae
- Oriolus kundoo
- Oriolus larvatus
- Oriolus melanotis
- Oriolus mellianus
- Oriolus monacha
- Oriolus nigripennis
- Oriolus oriolus
- Oriolus percivali
- Oriolus phaeochromus
- Oriolus sagittatus
- Oriolus steerii
- Oriolus szalayi
- Oriolus tenuirostris
- Oriolus traillii
- Oriolus xanthonotus
- Oriolus xanthornus
- Oristano
- Oristano (Sardinia)
- Oristano Sardinia, Italy
- Oriturus
- Oriturus superciliosus
- Orkney
- Orkney Islands


