• Oahu AmakihiChlorodrepanis flava
• Oahu ElepaioChasiempis ibidis
• Oak TitmouseBaeolophus inornatus
• Oaxaca SparrowAimophila notosticta
• Ocellated AntbirdPhaenostictus mcleannani
• Ocellated PiculetPicumnus dorbignyanus
• Ocellated PoorwillNyctiphrynus ocellatus
• Ocellated QuailCyrtonyx ocellatus
• Ocellated TapaculoAcropternis orthonyx
• Ocellated ThrasherToxostoma ocellatum
• Ocellated TurkeyMeleagris ocellata
• Ocellated WoodcreeperXiphorhynchus ocellatus
• Ochraceous AttilaAttila torridus
• Ochraceous BulbulAlophoixus ochraceus
• Ochraceous PiculetPicumnus limae
• Ochraceous WrenTroglodytes ochraceus
• Ochre-backed WoodpeckerCeleus ochraceus
• Ochre-bellied DoveLeptotila ochraceiventris
• Ochre-bellied FlycatcherMionectes oleagineus
• Ochre-breasted AntpittaGrallaricula flavirostris
• Ochre-breasted BrushfinchAtlapetes semirufus
• Ochre-breasted Foliage-gleanerAnabacerthia lichtensteini
• Ochre-breasted PipitAnthus nattereri
• Ochre-breasted TanagerChlorothraupis stolzmanni
• Ochre-browed ThistletailAsthenes coryi
• Ochre-cheeked SpinetailSynallaxis scutata
• Ochre-collared PiculetPicumnus temminckii
• Ochre-crowned GreenletTunchiornis ochraceiceps
• Ochre-faced Tody-FlycatcherPoecilotriccus plumbeiceps
• Ochre-flanked TapaculoEugralla paradoxa
• Ochre-fronted AntpittaGrallaricula ochraceifrons
• Ochre-lored FlatbillTolmomyias flaviventris
• Ochre-marked ParakeetPyrrhura cruentata
• Ochre-rumped AntbirdDrymophila ochropyga
• Ochre-rumped BuntingEmberiza yessoensis
• Ochre-striped AntpittaGrallaria dignissima
• Ochre-throated Foliage-gleanerAutomolus ochrolaemus
• OilbirdSteatornis caripensis
• Oleaginous HemispingusSphenopsis frontalis
• Olivaceous ElaeniaElaenia mesoleuca
• Olivaceous GreenletHylophilus olivaceus
• Olivaceous PiculetPicumnus olivaceus
• Olivaceous SchiffornisSchiffornis olivacea
• Olivaceous SiskinSpinus olivaceus
• Olivaceous WoodcreeperSittasomus griseicapillus
• Olive Bee-eaterMerops superciliosus
• Olive BulbulIole viridescens
• Olive FinchArremon castaneiceps
• Olive Long-tailed CuckooCercococcyx olivinus
• Olive OropendolaPsarocolius bifasciatus
• Olive SparrowArremonops rufivirgatus
• Olive SpinetailCranioleuca obsoleta
• Olive SunbirdCyanomitra olivacea
• Olive ThrushTurdus olivaceus
• Olive WarblerPeucedramus taeniatus
• Olive WhistlerPachycephala olivacea
• Olive-backed EuphoniaEuphonia gouldi
• Olive-backed Foliage-gleanerAutomolus infuscatus
• Olive-backed OrioleOriolus sagittatus
• Olive-backed PipitAnthus hodgsoni
• Olive-backed TailorbirdOrthotomus sepium
• Olive-backed TanagerMitrospingus oleagineus
• Olive-backed WoodcreeperXiphorhynchus triangularis
• Olive-bellied SunbirdCinnyris chloropygius
• Olive-capped WarblerSetophaga pityophila
• Olive-chested FlycatcherMyiophobus cryptoxanthus
• Olive-crowned CrescentchestMelanopareia maximiliani
• Olive-crowned YellowthroatGeothlypis semiflava
• Olive-gray SaltatorSaltator olivascens
• Olive-green CamaropteraCamaroptera chloronota
• Olive-green TanagerOrthogonys chloricterus
• Olive-green TyrannuletPhylloscartes virescens
• Olive-sided FlycatcherContopus cooperi
• Olive-spotted HummingbirdTalaphorus chlorocercus
• Olive-streaked FlycatcherMionectes olivaceus
• Olive-throated ParakeetEupsittula nana
• Olive-tree WarblerHippolais olivetorum
• Olive-winged BulbulPycnonotus plumosus
• OmaoMyadestes obscurus
• One-colored BecardPachyramphus homochrous
• Opal-crowned ManakinLepidothrix iris
• Opal-rumped TanagerTangara velia
• Orange Ground-ThrushGeokichla gurneyi
• Orange MinivetPericrocotus flammeus
• Orange OrioleIcterus auratus
• Orange River FrancolinScleroptila gutturalis
• Orange River White-eyeZosterops pallidus
• Orange-backed TroupialIcterus croconotus
• Orange-banded FlycatcherNephelomyias lintoni
• Orange-banded ThrushGeokichla peronii
• Orange-bellied AntwrenTerenura sicki
• Orange-bellied EuphoniaEuphonia xanthogaster
• Orange-bellied FlowerpeckerDicaeum trigonostigma
• Orange-bellied Fruit-DovePtilinopus iozonus
• Orange-bellied LeafbirdChloropsis hardwickii
• Orange-bellied ManakinLepidothrix suavissima
• Orange-billed Nightingale-ThrushCatharus aurantiirostris
• Orange-billed SparrowArremon aurantiirostris
• Orange-breasted BuntingPasserina leclancherii
• Orange-breasted FalconFalco deiroleucus
• Orange-breasted Forest RobinStiphrornis erythrothorax
• Orange-breasted FruiteaterPipreola jucunda
• Orange-breasted Green-PigeonTreron bicinctus
• Orange-breasted SunbirdAnthobaphes violacea
• Orange-breasted ThornbirdPhacellodomus ferrugineigula
• Orange-breasted TrogonHarpactes oreskios
• Orange-cheeked ParrotPyrilia barrabandi
• Orange-cheeked WaxbillEstrilda melpoda
• Orange-chinned ParakeetBrotogeris jugularis
• Orange-collared ManakinManacus aurantiacus
• Orange-crested FlycatcherMyiophobus phoenicomitra
• Orange-crowned ManakinHeterocercus aurantiivertex
• Orange-crowned OrioleIcterus auricapillus
• Orange-crowned WarblerLeiothlypis celata
• Orange-eyed FlatbillTolmomyias traylori
• Orange-eyed ThornbirdPhacellodomus erythrophthalmus
• Orange-footed MegapodeMegapodius reinwardt
• Orange-fronted BarbetCapito squamatus
• Orange-fronted ParakeetEupsittula canicularis
• Orange-fronted PlushcrownMetopothrix aurantiaca
• Orange-headed TanagerThlypopsis sordida
• Orange-headed ThrushGeokichla citrina
• Orange-throated LongclawMacronyx capensis
• Orange-throated TanagerWetmorethraupis sterrhopteron
• Orange-winged AmazonAmazona amazonica
• Orchard OrioleIcterus spurius
• Oriental Bay-OwlPhodilus badius
• Oriental CuckooCuculus optatus
• Oriental DarterAnhinga melanogaster
• Oriental GreenfinchChloris sinica
• Oriental Honey-buzzardPernis ptilorhynchus
• Oriental MagpiePica serica
• Oriental Magpie-RobinCopsychus saularis
• Oriental Pied-HornbillAnthracoceros albirostris
• Oriental PratincoleGlareola maldivarum
• Oriental Reed WarblerAcrocephalus orientalis
• Oriental Scops-OwlOtus sunia
• Oriental SkylarkAlauda gulgula
• Oriental Turtle-DoveStreptopelia orientalis
• Oriente WarblerTeretistris fornsi
• Orinocan SaltatorSaltator orenocensis
• Orinoco GooseOressochen jubatus
• Orinoco SofttailThripophaga cherriei
• Oriole BlackbirdGymnomystax mexicanus
• Oriole WarblerHypergerus atriceps
• Oriole WhistlerPachycephala orioloides
• Ornate FlycatcherMyiotriccus ornatus
• Ornate Hawk-EagleSpizaetus ornatus
• Ornate StipplethroatEpinecrophylla ornata
• Ornate TinamouNothoprocta ornata
• Ortolan BuntingEmberiza hortulana
• OspreyPandion haliaetus
• Oustalet's TyrannuletPhylloscartes oustaleti
• OvenbirdSeiurus aurocapilla
• Owston's TitSittiparus owstoni