- Oasis Hummingbird
- Oaxaca Hummingbird
- Oaxaca Sparrow
- Oberholser's Fruit Dove
- Obscure Berrypecker
- Ocellated Antbird
- Ocellated Piculet
- Ocellated Poorwill
- Ocellated Tapaculo
- Ocellated Thrasher
- Ocellated Turkey
- Ocellated Woodcreeper
- Ochraceous-breasted Flycatcher
- Ochraceous Bulbul
- Ochre-bellied Boobook
- Ochre-bellied Flycatcher
- Ochre-breasted Antpitta
- Ochre-breasted Tanager
- Ochre-collared Monarch
- Ochre-faced Tody-Flycatcher
- Ochre-lored Flatbill
- Ochre-naped Ground Tyrant
- Oilbird
- Olivaceous Flycatcher
- Olivaceous Piculet
- Olivaceous Woodcreeper
- Olive-backed Flowerpecker
- Olive-backed Oriole
- Olive-backed Pipit
- Olive-backed Quail-Dove
- Olive-backed Tailorbird
- Olive-backed Woodcreeper
- Olive-backed Woodpecker
- Olive-bellied Sunbird
- Olive-breasted Greenbul
- Olive-capped Flowerpecker
- Olive-crowned Yellowthroat
- Olive-flanked Ground Robin
- Olive-green Camaroptera
- Olive-grey Saltator
- Olive-headed Lorikeet
- Olive-naped Weaver
- Olive-sided Flycatcher
- Olive-striped Flycatcher
- Olive-throated Parakeet
- Olive-winged Bulbul
- Olive Bee-eater
- Olive Bushshrike
- Olive Honeyeater
- Olive Ibis
- Olive Long-tailed Cuckoo
- Olive Oropendola
- Olive Sunbird
- Olive Thrush
- Olive Warbler
- Olive Whistler
- Olive Woodpecker
- One-colored Becard
- Orange-backed Troupial
- Orange-backed Woodpecker
- Orange-bellied Euphonia
- Orange-bellied Flowerpecker
- Orange-bellied Fruit Dove
- Orange-bellied Leafbird
- Orange-bellied Parrot
- Orange-billed Babbler
- Orange-billed Lorikeet
- Orange-billed Nightingale-Thrush
- Orange-billed Sparrow
- Orange-breasted Bunting
- Orange-breasted Bushshrike
- Orange-breasted Fruiteater
- Orange-breasted Green Pigeon
- Orange-breasted Sunbird
- Orange-breasted Trogon
- Orange-breasted Waxbill
- Orange-cheeked Honeyeater
- Orange-cheeked Waxbill
- Orange-chinned Parakeet
- Orange-collared Manakin
- Orange-crowned Oriole
- Orange-crowned Warbler
- Orange-eared Tanager
- Orange-eyed Flatbill
- Orange-eyed Thornbird
- Orange-footed Scrubfowl
- Orange-fronted Fruit Dove
- Orange-fronted Parakeet
- Orange-fronted Plushcrown
- Orange-headed Thrush
- Orange-sided Thrush
- Orange-spotted Bulbul
- Orange-tufted Spiderhunter
- Orange-winged Amazon
- Orange-winged Pytilia
- Orange Chat
- Orange Ground Thrush
- Orange Minivet
- Orange River White-eye
- Orange Weaver
- Orangequit
- Orchard Oriole
- Oriental Bay Owl
- Oriental Darter
- Oriental Dollarbird
- Oriental Magpie
- Oriental Magpie-Robin
- Oriental Pied Hornbill
- Oriental Plover
- Oriental Pratincole
- Oriental Reed Warbler
- Oriental Scops Owl
- Oriental Skylark
- Oriental Turtle Dove
- Orinoco Goose
- Orinoco Saltator
- Oriole Blackbird
- Oriole Warbler
- Ornate Flycatcher
- Ornate Hawk-Eagle
- Ornate Melidectes
- Ornate Stipplethroat
- Ornate Sunbird
- Ornate Tinamou
- Ortolan Bunting
- Osprey
- Osprey (cristatus)
- Otago Shag
- Oustalet's Sunbird
- Ovenbird