O
- Old Baptist Union
- Old Catholic Church
- Old Lutherans
- Old Testament
- Olympianus of Byzantium
- Oneness Pentecostalism
- Onesimus
- Open Brethren
- Oriental Orthodoxy
- Origen
- Origen of Alexandria
- Orthodox Church in America
- Orthodox Tewahedo biblical canon
- Ottoman Empire
- Our Lady of Guadalupe
- Outline of Christian theology
- Outline of religion
- Outline of spirituality