- Octavia, Empress, consort of Nero, Emperor of Rome, approximately 42-621
- Oedipus (Greek mythological figure)1
- On style (Demetrius)1
- Orden de Santiago1
- Order of the Garter1
- Ordnance--Manufacture1
- Oria, Johannes d'1
- Orlandini, Stefano1
- Orso, Saint1
- Orthodox Eastern Church1
- Orto, Giovanni dall'1
- Ospedale degli Innocenti (Florence, Italy)1
- Paffiere, Arcangelo1
- Paffiere, Matteo1
- Pagani, Angiola1
- Pagliaiolo, Cenni1
- Painted bindings1
- Paltonibus, Antonio de1
- Pamphlets1
- Pannilini, Francesco1