- Oak Apple Gall Wasp
- Oak Artichoke Gall Wasp
- Oak Bush-cricket
- Oak Eggar
- Oak Marble Gall Wasp
- Oak Spider
- Oak Titmouse
- Oblique Streaktail
- Oblique-banded Leafroller Moth
- Obscure Bird Grasshopper
- Ochre Sea Star
- Ocola Skipper
- Odorous House Ant
- Oedemera femorata
- Oedemera podagrariae
- Old World Swallowtail
- Oleander Aphid
- Oleander Hawkmoth
- Olive-sided Flycatcher
- Opalescent Nudibranch
- Orange Bluet
- Orange Footman
- Orange Ladybird
- Orange Sea Cucumber
- Orange Sulphur
- Orange Swift
- Orange Wing
- Orange-belted Leafwalker
- Orange-crowned Warbler
- Orange-headed Callima Moth
- Orange-legged Furrow Bee
- Orange-spined Drone Fly
- Orange-tip
- Orca
- Orchard Orbweaver
- Orchard Oriole
- Orchard Swallowtail
- Oriental Beetle
- Oriental Cockroach
- Oriental Greenfinch
- Oriental Honey-buzzard
- Oriental Magpie
- Oriental Magpie-Robin
- Oriental Pied Hornbill
- Oriental Turtle-Dove
- Ornate Bella Moth
- Ornate Checkered Beetle
- Ornate Sunbird
- Ornate Tree Lizard
- Ornate Wrasse
- Osprey
- Ovenbird