Encyclopedias, Almanacs, Transcripts, and Maps
- Old World Porcupines (Hystricidae)
- Old World Sucker-Footed Bats (Myzopodidae)
- Old World Warblers (Sylviidae)
- Old-Growth Forest
- Oldenberg, Hermann
- Oligochaeta (Earthworms)
- Olmec Religion
- Olms and Mudpuppies (Proteidae)
- Olmsted, Frederick Law, Sr. (1822 – 1903) American Landscape Designer
- Oṃ
- Omayyads
- Omophagia
- Ōmotokyō
- on-line publishing
- Ongon
- Onmyōdō
- Onychophora (Onychophorans, velvet worms, and peripatus)
- Open Marsh Water Management
- Open System
- opencast mining