Encyclopedias, Almanacs, Transcripts, and Maps
- 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)
- oxbow lake
- Oxidation Reduction Reactions
- Oxidizing Agent
- Oxtoby, Willard G.
- oystercatcher
- Oystercatchers (Haematopodidae)