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- Barack Obama, 44th President of the United States, US Senator from Illinois, Illinois State Senator
- Michelle Obama, First Lady of the United States, wife of Barack Obama
- Hugh O'Brian, actor, The Life and Legend of Wyatt Earp
- Bob Ociepka, assistant coach for the Portland Trail Blazers
- Donald O'Connor, dancer, singer, actor, Singin' in the Rain
- Kevin J. O'Connor, actor, The Beast, The Others, The Mummy
- Bill Odenkirk, comedy writer, actor, producer, Mr. Show, The Simpsons
- Bob Odenkirk, actor, comedian, writer, director, Breaking Bad, Mr. Show, Tim and Eric Awesome Show, Great Job!
- Chris O'Donnell, actor, NCIS: Los Angeles, Batman Forever, Batman & Robin
- William Butler Ogden, politician (Democrat), businessman, first Mayor of Chicago
- Richard James Oglesby, politician (Republican), U.S. Army General, U.S. Senator, Governor of Illinois
- Gail O'Grady, actress, American Dreams, NYPD Blue, Hellcats
- Matt O'Leary, actor
- Ken Olin, actor, director, producer, Thirtysomething, Brothers & Sisters, L.A. Doctors
- Zak Orth, actor, Revolution
- Diana Oughton, student activist, member of The Weathermen
Read more about this topic: List Of People From Illinois