Encyclopedias, Almanacs, Transcripts, and Maps
- Octodonts (Octodontidae)
- Odén, Svante Swedish Agricultural Scientist
- Odonata (Dragonflies and Damselflies)
- Odor Control
- Odum, Dr. Eugene P. (1913 – ) American Ecologist
- Oersted, Hans Christian (1777–1851)
- Off-Road Vehicles
- Office of Civilian Radioactive Waste Management
- Office of Surface Mining
- 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
- Olaf II, Saint
- Olaf the Holy