O
- Old Believers
- Old calendarists
- Old Church Slavonic
- Omophorion
- Optina Monastery
- Oriental Orthodoxy
- Orientalium Ecclesiarum
- Orthodox Church of Antioch
- Orthodox Church of Constantinople
- Orthodox Church of Jerusalem
- Orthodox Church in America
- Orthodox Church in Italy
- Orthodox Ohrid Archbishopric
Read more about this topic: Index Of Eastern Christianity-related Articles