- O'Casey, Sean – Irish dramatist
- O'Connor, Flannery – American author
- O'Faolain, Sean – Irish short story author
- O'Hara, Kane
- O'Hara, Mary
- O'Keefe, Caldwell
- O'Keeffe, Georgia – American painter of Southwestern motifs
- O'Malley, Austin
- O'Reilly, Jane
- O'Rourke, P. J. – American humorist & political commentator
- O'Shaughnessy, Arthur
- Oates, Captain Lawrence
- Oates, Joyce Carol – American author
- Obama, Barack – 44th President of United States
- Ockham, William – English Scholastic philosopher
- Octavius, Augustus
- Ogburn, Charlton
- Ogilvy, David M.
- Olds, Sharon
| - Olinghouse, Lane
- Oliphant, Matthew
- Olsen, Ken – American computer engineer & industrialist
- Oppenheim, E. Phillips
- Oppenheim, James
- Oppenheimer, J. Robert – American administrator & astrophysicist
- Orben, Robert
- Orczy, Baroness Emmuska
- Orman, Suze
- Orton, Joe
- Orwell, George – English essayist, novelist, & satirist
- Osler, Sir William – British (Canadian-born) physician
- Otis, James – American politician in American Revolution
- Overbury, Thomas – English courtier & poet
- Ovid – Roman poet
- Owen, Roderic
- Oxenham, John
- Ozick, Cynthia
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