Encyclopedias, Almanacs, Transcripts, and Maps
- Orientales
- Orientalists, International Congress of
- Oriente (Bolivia)
- Oriente (Ecuador)
- Origen (c. 185–253)
- Origins of Iron Production
- Orinoco River
- Orixás
- ORKNEY AND SHETLAND DIALECTS
- Orlich Bolmarcich, Francisco José (1907–1969)
- Oro, Justo Santa María de (1772–1836)
- Orobio de Castro, Isaac (c. 1617–1687)
- Orochi
- Orok
- Orokaiva
- Orokolo
- Oroqen
- Orozco y Berra, Manuel (1818–1881)
- Orozco, José Clemente (1883–1949)
- Orozco, Olga (1920–1999)