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- Bernard Michael O'Brien, New Zealand Jesuit priest and philosopher.
- Joseph T. O'Callahan, U.S. Navy chaplain; awarded Medal of Honor
- Saint John Ogilvie, Scottish martyr
- Joseph A. O'Hare, former president of Fordham University and chairman of the New York City Charter Revision Commission and the first New York City Campaign Finance Board
- Gian Paolo Oliva, Italian 11th Superior General of the Society of Jesus
- William O'Malley, author and actor (played Father Joe Dyer in The Exorcist)
- Walter J. Ong, American cultural historian and spiritual writer
- Blessed Peter Onizuko, Japanese martyr burned at the stake during 17th century persecutions in Japan
- Saint Nicholas Owen (martyr), Martyr Saint of England and Wales
Read more about this topic: List Of Jesuits