Encyclopedias, Almanacs, Transcripts, and Maps
- Obregón, José (1832–1902)
- Ocampo, Melchor (1813–1861)
- Ocampo, Victoria (1890–1979)
- Occidental Petroleum
- Occitans
- Ochoa Reyes, Lorena (1981–)
- Ochs, Philip David ("Phil")
- Ockhamism
- Odell, Allan Gilbert
- Odio, Eunice (1919–1974)
- Odría, Manuel Apolinario (1897–1974)
- Oduber Quirós, Daniel (1921–1992)
- Oerter, Al(fred) Adolph, Jr.
- OFFICIAL ENGLISH
- OFFICIAL LANGUAGE
- Ogan-Besemah
- Ogé, Jacques Vicente (1755–1791)
- Ogoni
- Oidor
- Oil Embargo (1973–1974)