- Order of the Golden Fleece1
- Orders of knighthood and chivalry2
- Ordinatio (Duns Scotus, John)1
- Ordnance--Manufacture2
- Oria, Johannes d'1
- Orphans1
- Orphans in literature1
- Orthodox Eastern Church5
- 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 syllabi8
- Oviedo family1
- Oviedo, Francisco de, 1728-1
- Oviedo, Gregorio de, 1721-17961
- Oviedo, Nicolás de, 1725-1
- Pablo de Santa Maria, Bishop of Burgos, approximately 1351-14351
- Pacheco de Silva, Juana1