- Orden de Alcántara1
- Order of the Golden Fleece1
- Orders of knighthood and chivalry1
- Ordinances, Municipal1
- Ordnance--Manufacture1
- Organon (Aristotle)1
- Oria, Johannes d'1
- Oriental rites (Catholic Church)1
- Orlandini, Stefano1
- Orphans in literature1
- Orso, Saint1
- Orta, Garcia de, active 16th century1
- Ortiz family1
- Orto, Giovanni dall'1
- Ospedale degli Innocenti (Florence, Italy)1
- Ottava rimas, Spanish1
- Oviedo family1
- Oviedo, Francisco de, 1728-1
- Oviedo, Gregorio de, 1721-17961
- Oviedo, Nicolás de, 1725-1