Encyclopedias, Almanacs, Transcripts, and Maps
- Oldest Discovered Burial Site
- Old–Growth Forests
- Ole Christensen Römer
- Ole Christensen Römer
- Ole-Johan Dahl
- Olga Oleinik
- Olga Taussky-Todd
- Olga Yamshchikova
- Oligohydramnios sequence
- Oligohydramnios Sequence
- Olive Clio Hazlett
- Oliveto Citra
- Olivier de Serres
- Olivier de Serres
- Olivopontocerebellar Atrophy
- Olms and Mudpuppies: Proteidae
- Olof Rudbeck
- Olympiodorus of Alexandria
- Omar Khayyam and the Solution of Cubic Equations
- Omarr, Sydney (1926-)