- Oceanodroma markhami
- Oceanodroma matsudairae
- Oceanodroma melania
- Oceanodroma microsoma
- Oceanodroma monorhis
- Oceanodroma monteiroi
- Oceanodroma socorroensis
- Oceanodroma tethys
- Oceanodroma tristrami
- Ocellated Antbird
- Ocellated Crake
- Ocellated Laughingthrush
- Ocellated Piculet
- Ocellated Poorwill
- Ocellated Quail
- Ocellated Tapaculo
- Ocellated Thrasher
- Ocellated Turkey
- Ocellated Woodcreeper
- Ochetorhynchus
- Ochetorhynchus andaecola
- Ochetorhynchus certhioides
- Ochetorhynchus harterti
- Ochetorhynchus melanurus
- Ochetorhynchus phoenicurus
- Ochetorhynchus ruficaudus
- Ochraceous-breasted Flycatcher
- Ochraceous-headed Scimitar-Babbler
- Ochraceous-headed Scimitar Babbler
- Ochraceous Attila
- Ochraceous Bearded Bulbul
- Ochraceous Bulbul
- Ochraceous Hemispingus
- Ochraceous Pewee
- Ochraceous Piculet
- Ochraceous Wren
- Ochre-backed Woodpecker
- Ochre-bellied Boobook
- Ochre-bellied Dove
- Ochre-bellied Flycatcher
- Ochre-bellied Foliage-gleaner
- Ochre-bellied Hawk-Owl
- Ochre-bellied Hawk Owl
- Ochre-breasted Antpitta
- Ochre-breasted Brush-Finch
- Ochre-breasted Brush Finch
- Ochre-breasted Brushfinch
- Ochre-breasted Catbird
- Ochre-breasted Foliage-gleaner
- Ochre-breasted Francolin
- Ochre-breasted Pipit
- Ochre-breasted Tanager
- Ochre-breasted brush-finch
- Ochre-browed Thistletail
- Ochre-cheeked Spinetail
- Ochre-collared Monarch
- Ochre-collared Piculet
- Ochre-faced Tody-Flycatcher
- Ochre-faced Tody Flycatcher
- Ochre-flanked Tapaculo
- Ochre-fronted Antpitta
- Ochre-headed Flycatcher
- Ochre-lored Flatbill
- Ochre-marked Parakeet
- Ochre-naped Ground-Tyrant
- Ochre-naped Ground Tyrant
- Ochre-rumped Antbird
- Ochre-rumped Bunting
- Ochre-striped Antpitta
- Ochre-tailed Flycatcher
- Ochre Oriole
- Ochre collared Monarch
- Ochthoeca
- Ochthoeca cinnamomeiventris
- Ochthoeca cinnamomeiventris angustifasciata
- Ochthoeca diadema
- Ochthoeca diadema jesupi
- Ochthoeca frontalis
- Ochthoeca fumicolor
- Ochthoeca fumicolor superciliosa
- Ochthoeca jelskii
- Ochthoeca leucophrys
- Ochthoeca nigrita
- Ochthoeca oenanthoides
- Ochthoeca piurae
- Ochthoeca pulchella
- Ochthoeca rufipectoralis
- Ochthoeca spodionota
- Ochthoeca thoracica
- Ochthornis
- Ochthornis littoralis
- Ocreatus
- Ocreatus addae
- Ocreatus peruanus
- Ocreatus underwoodii
- Oculocincta
- Oculocincta squamifrons
- Ocyalus
- Ocyalus latirostris
- Ocyceros
- Ocyceros birostris
- Ocyceros gingalensis
- Ocyceros griseus
- Ocyphaps
- Ocyphaps lophotes
- Ocyphaps lophotes lophotes
- Odedi
- Odedi Bush-warbler
- Odontophoridae
- Odontophorus
- Odontophorus atrifrons
- Odontophorus balliviani
- Odontophorus capueira
- Odontophorus columbianus
- Odontophorus dialeucos
- Odontophorus erythrops
- Odontophorus gujanensis
- Odontophorus guttatus
- Odontophorus hyperythrus
- Odontophorus leucolaemus
- Odontophorus melanonotus
- Odontophorus melanotis
- Odontophorus speciosus
- Odontophorus stellatus
- Odontophorus strophium
- Odontorchilus
- Odontorchilus branickii
- Odontorchilus cinereus
- Odontospiza
- Odontospiza caniceps
- Odontospiza griseicapilla
- Oeanthe Oenanthe
- Oedistoma
- Oedistoma iliolophum
- Oedistoma iliolophus
- Oedistoma pygmaeum
- Oeland
- Oena
- Oena capensis
- Oenanthe
- Oenanthe Alboniger
- Oenanthe Cypriaca
- Oenanthe Oenanthe
- Oenanthe albifrons
- Oenanthe alboniger
- Oenanthe albonigra
- Oenanthe bifasciata
- Oenanthe bottae
- Oenanthe chrysopygia
- Oenanthe cypriaca
- Oenanthe deserti
- Oenanthe dubia
- Oenanthe familiaris
- Oenanthe familiaris familiaris
- Oenanthe finschii
- Oenanthe fusca
- Oenanthe halophila
- Oenanthe heuglini
- Oenanthe hispanica
- Oenanthe isabellina
- Oenanthe leucopyga
- Oenanthe leucura
- Oenanthe lugens
- Oenanthe lugens basalti
- Oenanthe lugentoides
- Oenanthe lugubris
- Oenanthe lugubris schalowi
- Oenanthe melanoleuca
- Oenanthe melanura
- Oenanthe moesta
- Oenanthe monacha
- Oenanthe monticola
- Oenanthe oenanthe
- Oenanthe oenanthe leucorhoa
- Oenanthe oenanthe libanotica
- Oenanthe phillipsi
- Oenanthe picata
- Oenanthe picata opistholeuca
- Oenanthe picata picata
- Oenanthe pileata
- Oenanthe pileata livingstonii
- Oenanthe pleschanka
- Oenanthe schalowi
- Oenanthe scotocerca
- Oenanthe xanthoprymna
- Oenanthe xanthoprymna chrysopygia
- Oenositta frontalis
- Oenositta oenochlamys
- Oenositta solangiae
- Oevre Dividal National Park
- Oevre Dividal National Park, Norway
- Ogden Reservoir
- Ogden Reservoir(Yorkshire)
- Ogden Resouvoir(Yorkshire)
- Ogea Flycatcher
- Ogea Monarch
- Ogle's Laughingthrush
- Ognorhynchus
- Ognorhynchus icterotis
- Ogston Reservoir
- Ogston Reservoir (Derbyshire)
- Ohio
- Oilbird
- Okarito Brown Kiwi
- Okarito Kiwi
- Okhla Bird Sanctuary
- Okinawa Rail
- Okinawa Woodpecker
- Oklahoma
- Oland
- Old Hall Marshes
- Old Hall Marshes RSPB Reserve (Essex)
- Old Man Bird
- Old Moor
- Old Women Bird
- Oldsquaw
- Oleaginous Hemispingus
- Oleaginous Tanager
- Olivaceous Alseonax
- Olivaceous Berrypecker
- Olivaceous Bulbul
- Olivaceous Cormorant
- Olivaceous Elaenia
- Olivaceous Flatbill
- Olivaceous Flycatcher
- Olivaceous Greenlet
- Olivaceous Leaf-Warbler
- Olivaceous Piculet
- Olivaceous Piha
- Olivaceous Schiffornis
- Olivaceous Siskin
- Olivaceous Thornbill
- Olivaceous Thrush
- Olivaceous Warbler
- Olivaceous Whistler
- Olivaceous Woodcreeper
- Olive-backed Euphonia
- Olive-backed Flowerpecker
- Olive-backed Foliage-gleaner
- Olive-backed Oriole
- Olive-backed Pipit
- Olive-backed Quail-Dove
- Olive-backed Quail Dove
- Olive-backed Sunbird
- Olive-backed Tailorbird
- Olive-backed Tanager
- Olive-backed Warbler
- Olive-backed Woodcreeper
- Olive-backed Woodpecker
- Olive-bellied Sunbird
- Olive-breasted Greenbul
- Olive-brown Bulbul
- Olive-brown Honeyeater
- Olive-brown Oriole
- Olive-capped Flowerpecker
- Olive-capped Warbler
- Olive-chested Flycatcher
- Olive-colored White-eye
- Olive-crowned Crescent-chest
- Olive-crowned Crescentchest
- Olive-crowned Flowerpecker
- Olive-crowned Sparrow
- Olive-crowned Yellowthroat
- Olive-faced Flatbill
- Olive-flanked Ground-Robin
- Olive-flanked Ground Robin
- Olive-flanked Robin-Chat
- Olive-flanked Robin Chat
- Olive-flanked Whistler
- Olive-green Camaroptera
- Olive-green Tanager
- Olive-green Tyrannulet
- Olive-headed Brush-Finch
- Olive-headed Brush Finch
- Olive-headed Brushfinch
- Olive-headed Greenbul
- Olive-headed Lorikeet
- Olive-headed Weaver
- Olive-headed brush-finch
- Olive-lored Oriole
- Olive-rumped Serin
- Olive-shouldered Parrot
- Olive-sided Flycatcher
- Olive-sided Fulvetta
- Olive-spotted Hummingbird
- Olive-streaked Honeyeater
- Olive-striped Flycatcher
- Olive-tailed Thrush
- Olive-tailed or Bassian Thrush
- Olive-throated Conure
- Olive-throated Parakeet
- Olive-tree Warbler
- Olive-tufted Flycatcher
- Olive-winged Bulbul
- Olive-yellow Flycatcher
- Olive-yellow Myzomela
- Olive-yellow Robin
- Olive Bee-eater
- Olive Black-eye
- Olive Brush-finch
- Olive Brush Finch
- Olive Bulbul
- Olive Bush-shrike
- Olive Bush Shrike
- Olive Bushshrike
- Olive Finch
- Olive Flycatcher
- Olive Flyrobin
- Olive Honeyeater
- Olive Ibis
- Olive Long-tailed Cuckoo
- Olive Manakin
- Olive Oropendola
- Olive Scrubwren
- Olive Sparrow
- Olive Spinetail
- Olive Straightbill
- Olive Striped Flycatcher
- Olive Sunbird
- Olive Tanager
- Olive Thickhead
- Olive Thrush
- Olive Tree-Pipit
- Olive Tree Warbler
- Olive Tufted Flycatcher
- Olive Warbler
- Olive Weaver-finch
- Olive Weaver Finch
- Olive Whistler
- Olive Woodpecker
- Olomao
- Olrog's Cinclodes
- Olrog's Gull
- Olson's Shearwater
- Oma'o
- Omak
- Oman
- Omani Owl
- Omao
- Omei Shan Liocichla
- Omei Warbler
- Omilteme Jay
- Omiltemi Jay
- Oncostoma
- Oncostoma cinereigulare


