Encyclopedias, Almanacs, Transcripts, and Maps
- Organs and Organ Systems
- Orientation of Strata
- Origen (Oregenes Adamantius)
- Orioles
- Orioles
- Oris (1954-)
- Ornithine transcarbamylase deficiency
- Ornithine Transcarbamylase Deficiency
- Ornithomancy
- Orthonectidans: Orthonectida
- Orthotolidine Solution
- Ortt, Felix (1866-1959)
- Osborn, Albert Sherman
- Osborn, Arthur W(alter) (1891-?)
- Osborn, Edward Collet (1909-1957)
- Oscar Zariski
- Oscillating Reactions
- Oscillating Reactions
- Oscillations
- Oscillations