Encyclopedias, Almanacs, Transcripts, and Maps
- Oviparous
- Ovolactovegetarianism
- Ovovegetarianism
- Ovoviviparous
- Ovoviviparous
- Owen, Alan Robert George (1919-)
- Owen, George Vale (1869-1931)
- Owen, Iris M
- Owens, Ted (1920-)
- Owlet-Nightjars: Aegothelidae
- Owls: Strigidae
- Owls: Strigiformes
- Oxazolindinediones
- Oxford Golden Dawn Occult Society
- Oxidation State
- Oxidation State
- Oxygen Cycle in Microorganisms
- Oystercatchers: Haematopodidae
- Oz Factor
- Ozanne, Charles E(ugene) (1865-1961)