• Obeah@ (11)
• Ocha@ (12)
• Old Baptist Union@ (0)
• Old Catholic Church of North America@ (3)
• Old Catholic Church of the Netherlands@ (2)
• Old Catholic Church of the United States@ (1)
• Old Catholics@ (47)
• One Holy Catholic and Apostolic Roman Church@ (1)

• Open Bible Standard Churches@ (8)
• Order of Melchizedek@ (5)
• Orthodox Bahá'í@ (8)
• Orthodox Christian Reformed Church@ (0)
• Orthodox Christianity@ (571)
• Orthodox Judaism@ (102)
• Orthodox Presbyterian Church@ (98)
• Osho@ (41)