- Oak Titmouse
- Oasis Hummingbird
- Oaxaca Sparrow
- Ocellated Antbird
- Ochraceous Bulbul
- Ochraceous Piculet
- Ochre-bellied Flycatcher
- Ochre-breasted Foliage-gleaner
- Ochre-cheeked Spinetail
- Ochre-collared Piculet
- Ochre-faced Tody-Flycatcher
- Ochre-lored Flatbill
- Ochre-naped Ground Tyrant
- Olivaceous Piculet
- Olivaceous Woodcreeper
- Olive-backed Pipit
- Olive-backed Woodcreeper
- Olive-capped Warbler
- Olive-crowned Yellowthroat
- Olive-green Tanager
- Olive-sided Flycatcher
- Olive-spotted Hummingbird
- Olive-throated Parakeet
- Olive Oropendola
- Olrog's Cinclodes
- Olrog's Gull
- Omao
- Orange-backed Troupial
- Orange-backed Woodpecker
- Orange-bellied Euphonia
- Orange-bellied Flowerpecker
- Orange-bellied Fruit Dove
- Orange-bellied Leafbird
- Orange-billed Nightingale-Thrush
- Orange-breasted Falcon
- Orange-cheeked Waxbill
- Orange-chinned Parakeet
- Orange-crowned Warbler
- Orange-fronted Barbet
- Orange-fronted Parakeet
- Orange-fronted Yellow Finch
- Orange-headed Tanager
- Orange-headed Thrush
- Orchard Oriole
- Oriental Darter
- Oriental Dollarbird
- Oriental Magpie-Robin
- Oriental Pied Hornbill
- Oriental Plover
- Oriental Pratincole
- Oriental Reed Warbler
- Oriental Scops Owl
- Oriental Stork
- Oriental Turtle Dove
- Oriente Warbler
- Ornate Flycatcher
- Ornate Hawk-Eagle
- Ornate Melidectes
- Ornate Sunbird
- Osprey
- Ovenbird