- Oberholser's Fruit Dove
- Obscure Berrypecker
- Ocellated Antbird
- Ochraceous Bulbul
- Ochraceous Pewee
- Ochre-bellied Boobook
- Ochre-bellied Flycatcher
- Ochre-breasted Antpitta
- Olivaceous Piculet
- Olive-backed Oriole
- Olive-backed Pipit
- Olive-backed Woodcreeper
- Olive-breasted Greenbul
- Olive-capped Flowerpecker
- Olive-crowned Flowerpecker
- Olive-striped Flycatcher
- Olive Bee-eater
- Olive Flyrobin
- Olive Sunbird
- Olive Thrush
- Olive Whistler
- Olrog's Cinclodes
- Olrog's Gull
- Orange-backed Troupial
- Orange-bellied Euphonia
- Orange-bellied Flowerpecker
- Orange-bellied Fruit Dove
- Orange-bellied Leafbird
- Orange-bellied Parrot
- Orange-billed Sparrow
- Orange-breasted Bushshrike
- Orange-breasted Sunbird
- Orange-breasted Trogon
- Orange-cheeked Waxbill
- Orange-chinned Parakeet
- Orange-collared Manakin
- Orange-fronted Fruit Dove
- Orange-fronted Hanging Parrot
- Orange-headed Thrush
- Orange Chat
- Orange Minivet
- Orange Weaver
- Orchard Oriole
- Oriental Cuckoo
- Oriental Darter
- Oriental Dollarbird
- Oriental Magpie-Robin
- Oriental Pied Hornbill
- Oriental Plover
- Oriental Reed Warbler
- Oriental Scops Owl
- Oriental Turtle Dove
- Ornate Melidectes
- Ortolan Bunting
- Osprey
- Otago Shag
- Ovenbird