- Oasis Hummingbird
- Oaxaca Hummingbird
- Oaxaca Sparrow
- Ocellated Antbird
- Ocellated Crake
- Ocellated Turkey
- Ocellated Woodcreeper
- Ochraceous Attila
- Ochraceous Piculet
- Ochre-backed Woodpecker
- Ochre-breasted Brushfinch
- Ochre-breasted Tanager
- Ochre-collared Piculet
- Ochre-fronted Antpitta
- Ochre-marked Parakeet
- Ochre-naped Ground Tyrant
- Olivaceous Elaenia
- Olive-backed Flowerpecker
- Olive-backed Foliage-gleaner
- Olive-backed Woodpecker
- Olive-bellied Sunbird
- Olive-crowned Flowerpecker
- Olive-crowned Yellowthroat
- Olive-green Camaroptera
- Olive-green Tanager
- Olive-naped Weaver
- Olive-streaked Flycatcher
- Olive Finch
- Olive Flyrobin
- Olive Oropendola
- Olive Sparrow
- Olive Straightbill
- Olive Sunbird
- Olive Tufted Flycatcher
- Olive Warbler
- One-colored Becard
- Opal-crowned Tanager
- Orange-backed Woodpecker
- Orange-bellied Euphonia
- Orange-bellied Flowerpecker
- Orange-billed Babbler
- Orange-billed Nightingale-Thrush
- Orange-breasted Bunting
- Orange-breasted Fruiteater
- Orange-breasted Thornbird
- Orange-cheeked Waxbill
- Orange-crowned Oriole
- Orange-eyed Flatbill
- Orange-eyed Thornbird
- Orange-fronted Barbet
- Orange-fronted Fruit Dove
- Orange-fronted Yellow Finch
- Orange-headed Tanager
- Orange-headed Thrush
- Orange-spotted Bulbul
- Orange-winged Amazon
- Orange River White-eye
- Orange Weaver
- Orchard Oriole
- Oriental Cuckooshrike
- Oriental Dollarbird
- Oriental Plover
- Oriental Turtle Dove
- Orinoco Goose
- Orinoco Piculet
- Ornate Flycatcher
- Ornate Fruit Dove
- Ornate Pitta
- Ornate Tinamou
- Ovambo Sparrowhawk