- Oaths2
- Ochoa family1
- Old age1
- Olive oil industry1
- Orden de Santiago2
- Order of the Golden Fleece1
- Orders of knighthood and chivalry2
- Ordinances, Municipal1
- Ordnance--Manufacture2
- Oria, Johannes d'1
- Orlandini, Stefano1
- Orta, Garcia de, active 16th century1
- Orthodox Eastern Church2
- Ortiz family1
- Orto, Giovanni dall'1
- Ospedale degli Innocenti (Florence, Italy)1
- Ospedale di San Bartolomeo (Florence, Italy)2
- Otto I, Holy Roman Emperor, 912-9731
- Ovando, Nicolás de, approximately 1460-15111
- Oviedo family1