Encyclopedias, Almanacs, Transcripts, and Maps
- oscillating Universe theory
- Osmeriformes (Smelts, Galaxiids, and Relatives)
- Osteoglossiformes (Bony Tongues and Relatives)
- Ostracoda (Mussel Shrimps)
- Ostriches (Struthionidae)
- Otherworld
- Othman
- Otomí Religion
- Otto, Nikolaus August (1832–1891)
- Otto, Rudolf
- Otto, Walter F.
- Our Common Future (Brundtland Report)
- Ouspensky, P. D.
- ouzel
- Ovenbirds (Furnariidae)
- Overhunting
- Overseas Development Administration
- Overuse
- Owlet-Nightjars (Aegothelidae)
- Owls (Strigidae)