Encyclopedias, Almanacs, Transcripts, and Maps
- Ocean Dumping Ban Act (1988)
- Ocean Farming
- Ocean Outfalls
- Ocean Thermal Energy Conversion
- Oceanic mythology
- Oceanic Religions
- Oceanic Religions: An Overview
- Oceanic Religions: Missionary Movements
- Oceanic Religions: New Religious Movements
- Oceans and Saltwater
- Octane Rating
- 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