Encyclopedias, Almanacs, Transcripts, and Maps
- Organized Labor Established
- Orgburo
- Origin and Diffusion
- Origins of the Cold War
- Origins of the Crusades
- Origins of the Industrial Revolution
- Orisha
- Oriskany, New York
- Orlov, Grigory Grigorievich
- Orphans and Foundlings
- Orphans and Orphanages
- Ortega y Gasset, José (1883–1955)
- Ortega, Katherine Davalos
- Orthodoxy, Greek
- Orthodoxy, Russian
- Ortiz, Alfonso
- Orwell, George (1903–1950)
- Osborne Judgment
- Osborne's (James River), Virginia
- Oscar Fingall O'Flahertie Wills Wilde