- Ocellated Piculet
- Ochraceous Attila
- Ochraceous Piculet
- Ochre-backed Woodpecker
- Ochre-breasted Antpitta
- Ochre-breasted Brushfinch
- Ochre-collared Monarch
- Ochre-lored Flatbill
- Ochre-rumped Antbird
- Ochre-rumped Bunting
- Oilbird
- Okinawa Rail
- Okinawa Woodpecker
- Olivaceous Piculet
- Olivaceous Woodcreeper
- Olive-backed Pipit
- Olive-backed Tailorbird
- Olive-backed Tanager
- Olive-backed Woodcreeper
- Olive-bellied Sunbird
- Olive-breasted Greenbul
- Olive-chested Flycatcher
- Olive-headed Lorikeet
- Olive-sided Flycatcher
- Olive Bee-eater
- Olive Manakin
- Olive Sunbird
- Olive Warbler
- Olive Woodpecker
- One-colored Becard
- Orange-backed Troupial
- Orange-bellied Antwren
- Orange-bellied Euphonia
- Orange-bellied Flowerpecker
- Orange-bellied Fruit Dove
- Orange-bellied Manakin
- Orange-billed Lorikeet
- Orange-breasted Bushshrike
- Orange-breasted Sunbird
- Orange-breasted Trogon
- Orange-breasted Waxbill
- Orange-cheeked Honeyeater
- Orange-cheeked Parrot
- Orange-cheeked Waxbill
- Orange-crested Flycatcher
- Orange-crowned Euphonia
- Orange-crowned Oriole
- Orange-crowned Warbler
- Orange-footed Scrubfowl
- Orange-fronted Fruit Dove
- Orange-fronted Hanging Parrot
- Orange-fronted Yellow Finch
- Orange-headed Thrush
- Orange-sided Thrush
- Orange-spotted Bulbul
- Orange-throated Tanager
- Orange-tufted Sunbird
- Orange-winged Amazon
- Orange-winged Pytilia
- Orange Fruit Dove
- Orange Minivet
- Orange River Francolin
- Orange Weaver
- Orchard Oriole
- Oriental Darter
- Oriental Dollarbird
- Oriental Hobby
- Oriental Magpie-Robin
- Oriental Pied Hornbill
- Oriental Pratincole
- Oriental Reed Warbler
- Oriental Scops Owl
- Oriental Skylark
- Oriental Turtle Dove
- Oriente Warbler
- Orinoco Goose
- Oriole Finch
- Oriole Warbler
- Oriole Whistler
- Ornate Flycatcher
- Ornate Melidectes
- Ortolan Bunting
- Osprey
- Otago Shag
- Oustalet's Sunbird
- Oustalet's Tyrannulet
- Ovambo Sparrowhawk
- Owston's Tit