Encyclopedias, Almanacs, Transcripts, and Maps
- O'Neill, Con, 1st earl of Tyrone
- O'Neill, Daniel
- O'Neill, Terence, Lord O'Neill of the Maine
- O'Toole, St Laurence
- Oakes, Richard
- OBAC Writers' Workshop
- Oblate Sisters of Providence
- Obradović, Dositej
- Obrok
- Obruchev, Nikolai Nikolayevich
- Obshchina
- Obstetrics and Gynecology
- Occult Philosophy
- Occupation, Military
- Occupations, East Asia
- Occupations, the Pacific
- Oceanic Exploration
- Ochakov crisis
- Octavians
- October General Strike of 1905