- oceanic crust, 1
- oceans, see seas
- oil, 1, 2, 3, 4
- olive, 1
- Ophioglossum vulgatum, 1
- optics, 1
- orang-utans, 1
- orbitals, 1
- organelles, 1, 2
- Ørsted, Hans Christian, 1
- outbreeding, 1
- ovaries, 1, 2
- ovum/ova, 1, 2, 3, 4, 5 n. 13
- oxidation, 1 n. 4
- oxygen, 1, 2, 3, 4, 5, 6, 7 nn. 2, 3, 4; 8 n. 4
- ozone, 1