Encyclopedias, Almanacs, Transcripts, and Maps
- Ongon
- Onmy?d?
- Onychophora (Onychophorans, velvet worms, and peripatus)
- Open Marsh Water Management
- Open System
- opencast mining
- opera buffa
- Ophidiiformes (Cusk-Eels and Relatives)
- Ophiuroidea (Brittle and Basket Stars)
- Opisthobranchia (Sea Slugs)
- Opisthocomiformes (Hoatzins)
- Opportunistic Organism
- opposition, leader of the
- Optical Effects
- Oracles
- Oral Torah
- Oral Tradition
- Orange, House of
- Order of Magnitude
- Orectolobiformes (Carpet Sharks)