- Ontology2
- Oratory1
- Orden de Santiago3
- Order of the Garter2
- Order of the Golden Fleece1
- Orders of knighthood and chivalry2
- Ordinances, Municipal1
- Ordnance--Manufacture2
- Orso, Saint1
- Orta, Garcia de, active 16th century1
- Orthodox Eastern Church3
- Ospedale degli Innocenti (Florence, Italy)1
- Ospedale di San Bartolomeo (Florence, Italy)2
- Otto I, Holy Roman Emperor, 912-9731
- Outlines and syllabi1
- Ovando, Nicolás de, approximately 1460-15111
- Oviedo family1
- Oviedo, Francisco de, 1728-1
- Oviedo, Gregorio de, 1721-17961
- Oviedo, Nicolás de, 1725-1