Index O
- Ocean see Sea
- Of Moths and Men
- Of Pandas and People
- Ohio
- Oktar, Adnan
- Old-Earth creationism
- Old Testament see Genesis
- Olduvai Gorge
- Omo fossils
- Omphalos
- Opportunity (Mars rover)
- Orangutans
- Oreopithecus bambolii
- Organic chemistry
- Orgasm
- Origin-of-life research see Abiogenesis
- Origin of Species
- Orrorin tugenensis
- Orthogenesis
- Osborn, Henry Fairfield
- Oscillating universe
- "Out of Africa" model
- Oxygen