Encyclopedias, Almanacs, Transcripts, and Maps
- Ottawa Agreement (1932)
- OTTAWA VALLEY
- Otto, Rudolf (1869–1937)
- Ottomanism
- Ottoni, Teofilo Benedito (1807–1869)
- Ouimet, Francis DeSales
- Oujda Group
- Ouled Sidi Cheikh
- Ouro Prêto
- Ousia
- Ouvidores
- Ovando Candía, Alfredo (1917–1982)
- Ovando, Nicolásde (1451–1518)
- Ovcharovo
- Overseas Council (Portugal)
- Oviedo y Valdés, Gonzalo Fernández (1478–1557)
- Ovimbundu
- Owen, G. E. L (1922–1982)
- Owens, James Cleveland ("Jesse")
- OXFORD ACCENT