Encyclopedias, Almanacs, Transcripts, and Maps
- Osborne, Thomas William ("Tom")
- Osirak
- Osman, House of
- Osório, Manuel Luís (1808–1879)
- Osorio, Oscar (1910–1969)
- Osorno
- Ospina Pérez, Mariano (1891–1976)
- Ospina Rodríguez, Mariano (1805–1885)
- Ospina, Pedro Nel (1858–1927)
- Ossetians
- Oswaldo Cruz Institute
- Otavalo
- Otero Vértiz, Gustavo Adolfo (1896–1958)
- Otero, Alejandro (1921–1990)
- Otero, Mariano (1817–1850)
- Other Minds
- Otomí
- Otomí of the Sierra
- Otomí of the Valley of Mezquital
- Ott, Mel(vin) Thomas