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- Conor Oberst (Bright Eyes) - American musician (Creighton Preparatory School, Omaha, Nebraska)
- Pat O'Brien - American actor (Marquette University High School and Marquette University, Milwaukee)
- Chris O'Donnell - Actor (Loyola Academy, Wilmette, IL and Boston College, Chestnut Hill, MA)
- Sir Anthony O'Reilly - Irish & Lions Rugby Union Teams and Businessman (Belvedere College, Dublin)
- Peter O'Toole - Irish actor
- Thomas P. 'Tip' O'Neill - American politician (Boston College, Chestnut Hill, Massachusetts)
- Paul Otellini - President of Intel Corporation (St. Ignatius College Preparatory, San Francisco)
- Charles Osgood - American media commentator (Fordham University)
- Tomas Osmeña - Filipino politician (Xavier University – Ateneo de Cagayan)
- Gerry Ottenheimer - Canadian politician and senator (Fordham University)
- Jacques Ozanam - French mathematician (Jesuit College in Rheims)
- Michael O'Shaughnessy - Chief Operating Officer, Walsh Jesuit, Stow, Ohio
Read more about this topic: List Of Alumni Of Jesuit Educational Institutions