Encyclopedias, Almanacs, Transcripts, and Maps
- Ogallala Aquifer
- Oil and Gas, Drilling for
- Oil and Gas, Exploration for
- Oil Drilling
- Oil Embargo
- Oilbirds (Steatornithidae)
- Okapis and Giraffes (Giraffidae)
- Okinawan Religion
- Okuninushi
- Ōkuninushi No Mikoto
- Olaf II, Saint
- Olaf the Holy
- Old World Deer (Cervinae)
- Old World Flycatchers (Muscicapidae)
- Old World Fruit Bats I (Pteropus)
- Old World Fruit Bats II (All Other Genera)
- Old World Leaf-Nosed Bats (Hipposideridae)
- Old World Monkeys I (Colobinae)
- Old World Monkeys II (Cercopithecinae)
- Old World Orioles and Figbirds (Oriolidae)