- Organon (Aristotle)1
- Oria, Johannes d'1
- Oriental rites (Catholic Church)1
- Origen1
- Orlandini, Stefano1
- Orphans1
- Orphans in literature1
- Orso, Saint1
- Orta, Garcia de, active 16th century1
- Orthodox Eastern Church14
- Ortiz family1
- Orto, Giovanni dall'1
- Ospedale degli Innocenti (Florence, Italy)1
- Ospedale di San Bartolomeo (Florence, Italy)2
- Otiyyot de-Rabbi Akiva2
- Ottava rimas, Spanish4
- Otto I, Holy Roman Emperor, 912-9731
- Outlines and syllabi12
- Ovando, Nicolás de, approximately 1460-15111
- Oviedo family1