- Occasional verse1
- Occultism--Religious aspects1
- Occultism--Religious aspects--Islam1
- Occupations1
- Occupations in literature1
- Ocean travel1
- Ochoa family1
- Octavia, Empress, consort of Nero, Emperor of Rome, approximately 42-621
- Octoechos (Orthodox Eastern Church)1
- Odes1
- Odes, Latin (Medieval and modern)1
- Odilo, Saint, Abbot of Cluny, 962-1048 or 10491
- Odo, Saint, Abbot of Cluny, approximately 879-9421
- Odyssey (Homer)1
- Oedipus (Greek mythological figure)1
- Offenses against religion1
- Olive oil1
- Olive oil industry1
- Omens--Religious aspects--Islam1
- On style (Demetrius)1