- Offenses against religion1
- Offertories (Music)1
- On style (Demetrius)1
- Oratory1
- Order of the Golden Fleece1
- Ordinances, Municipal1
- Orso, Saint1
- Orta, Garcia de, active 16th century1
- Ospedale degli Innocenti (Florence, Italy)1
- 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
- Paganelli, Roberto1
- Pain--Religious aspects--Christianity1
- Painted books1
- Pakhomiĭ Logofet, monk, active 15th century1