Encyclopedias, Almanacs, Transcripts, and Maps
- Oil Well Drilling
- Oil Well Drilling
- Oilbird: Steatornithidae
- Ointment, Witches'
- Ojai Foundation
- Okapi and Giraffe: Giraffidae
- Oki Yoga
- Oki, Masahiro (1921-)
- Oklahoma Bombing (1995 Bombing of Alfred P. Murrah Building)
- Old Hat Used for Raising the Devil
- Old World Flycatchers: Muscicapidae
- Old World Fruit Bats: Pteropodidae
- Old World Leaf-Nosed Bats: Hipposideridae
- Old World Monkeys: Cercopithecidae
- Old World Orioles and Figbirds: Oriolidae
- Old World Porcupines: Hystricidae
- Old World Sucker-Footed Bat: Myzopodidae
- Old World Warblers: Sylviidae
- Old, Walter G(orn) (1864-1929)
- Old-Growth Forests