| Bill O'Brien | British Politician |
| Conan O'Brien | American Entertainer |
| Dan O'Brien | American Athlete |
| Ed O'Brien | English Musician |
| Edna O'Brien | Irish Novelist |
| Flann O'Brien | Irish Writer |
| Kate O'Brien | Irish Novelist |
| Margaret O'Brien | American Actress |
| Richard O'Brien | English Actor |
| Sean O'Casey | Irish Playwright |
| Daniel O'Connell | Irish Politician |
| Jerry O'Connell | American Actor |
| William H. O'Connell | American Clergyman |
| Carroll O'Connor | American Actor |
| Donald O'Connor | |
| Flannery O'Connor | American Author |
| Frank O'Connor | Irish Author |
| Renee O'Connor | American Actress |
| Sinead O'Connor | Irish Musician |
| Nancy O'Dell | American Celebrity |
| Chris O'Donnell | American Actor |
| Rosie O'Donnell | American Comedian |
| Michael O'Donoghue | |
| Sean O'Faolain | Irish Writer |
| Paul O'Grady | English Comedian |
| Catherine O'Hara | Canadian Actress |
| Frank O'Hara | American Poet |
| John O'Hara | American Writer |
| Maureen O'Hara | Irish Actress |
| Georgia O'Keefe | American Artist |
| Georgia O'Keeffe | American Artist |
| Jean O'Leary | American Activist |
| Austin O'Malley | American Physicist |
| Alexander O'Neal | American Musician |
| Shaquille O'Neal | American Athlete |
| Tatum O'Neal | American Actress |
| Ed O'Neill | American Actor |
| Eugene O'Neill | American Dramatist |
| Jennifer O'Neill | American Actress |
| John O'Neill | |
| Paul O'Neill | American Politician |
| Thomas P. O'Neill | American Politician |
| Bill O'Reilly | American Journalist |
| Tim O'Reilly | |
| Heather O'Rourke | American Actress |
| P. J. O'Rourke | American Comedian |
| Arthur O'Shaughnessy | British Poet |
| Annette O'Toole | American Dancer |
| John O'Toole | Canadian Politician |
| Peter O'Toole | Irish Actor |
| Michael Oakeshott | English Philosopher |
| Ann Oakley | British Sociologist |
| Annie Oakley | American Celebrity |
| John Oates | American Musician |
| Johnny Oates | American Athlete |
| Barack Obama | American Politician |
| Olusegun Obasanjo | Nigerian Statesman |
| Merle Oberon | American Actress |
| Conor Oberst | American Musician |
| Hermann Oberth | German Scientist |
| David Obey | American Politician |
| David R. Obey | American Politician |
| Ric Ocasek | |
| Billy Ocean | British Musician |
| Ellen Ochoa | American Scientist |
| Arthur Ochs Sulzberger | American Publisher |
| Phil Ochs | American Musician |
| Bob Odenkirk | American Actor |
| Clifford Odets | American Playwright |
| William Odom | American Soldier |
| Kenzaburo Oe | Japanese Writer |
| Al Oerter | American Athlete |
| Joan of Arc | French Celebrity |
| St. Francis of Assisi | Italian Saint |
| Countess of Blessington | Irish Novelist |
| Earl of Chesterfield | English Statesman |
| Augustine of Hippo | Roman Saint |
| William of Occam | English Philosopher |
| Diogenes of Sinope | Greek Philosopher |
| Duke of Wellington | British Royalty |
| Duchess of Windsor | British Celebrity |
| David Ogden Stiers | American Actor |
| David Ogilvy | American Businessman |
| Sadaharu Oh | Japanese Athlete |
| Sandra Oh | Canadian Actress |
| Don Ohlmeyer | American Producer |
| Daniel Okrent | American Editor |
| Ben Okri | Nigerian Poet |
| Hakeem Olajuwon | Nigerian Athlete |
| Marvin Olasky | American Educator |
| Babatunde Olatunji | Nigerian Musician |
| Keith Olbermann | American Journalist |
| Claes Oldenburg | Swedish Sculptor |
| Bruce Oldfield | English Designer |
| Mike Oldfield | English Musician |
| John Oldham | American Celebrity |
| Will Oldham | American Musician |
| Gary Oldman | English Actor |
| Sharon Olds | American Poet |
| Larisa Oleynik | American Actress |
| Margaret Oliphant | Scottish Novelist |
| Mark Oliphant | Australian Scientist |
| Pat Oliphant | Australian Cartoonist |
| Tony Oliva | American Athlete |
| Andrew Oliver | American Politician |
| Barney Oliver | |
| Gene Oliver | American Athlete |
| James Oliver | British Politician |
| Jamie Oliver | British Celebrity |
| Mary Oliver | American Poet |
| Peter Oliver | English Artist |
| Susan Oliver | American Actress |
| Laurence Olivier | English Actor |
| Ashley Olsen | American Actress |
| Ken Olsen | American Businessman |
| Mary-Kate Olsen | American Actress |
| Merlin Olsen | American Athlete |
| Susan Olsen | American Actress |
| Tillie Olsen | American Novelist |
| Barbara Olson | American Journalist |
| Carl Olson | American Athlete |
| Charles Olson | American Poet |
| Johnny Olson | American Entertainer |
| Ted Olson | American Politician |
| John Olver | American Politician |
| Timothy Olyphant | American Actor |
| Mohammed Omar | Afghani Clergyman |
| Aristotle Onassis | Greek Businessman |
| Michael Ondaatje | Canadian Author |
| Jerry Only | |
| Yoko Ono | American Artist |
| John Opie | English Artist |
| George Oppen | American Poet |
| Harry Oppenheimer | |
| J. Robert Oppenheimer | American Physicist |
| A. R. Orage | British Editor |
| Jerry Orbach | American Actor |
| Robert Orben | American Entertainer |
| Roy Orbison | American Musician |
| Baroness Orczy | British Novelist |
| Carl Orff | German Composer |
| Origen | |
| Tony Orlando | American Musician |
| Suze Orman | American Author |
| Eugene Ormandy | American Musician |
| Julia Ormond | British Actress |
| Sarah Orne Jewett | American Author |
| Leo Ornstein | American Composer |
| Benjamin Orr | American Musician |
| Bobby Orr | Canadian Athlete |
| John B. Orr | |
| Leonard Orr | American Celebrity |
| Stacie Orrico | American Musician |
| Brian Orser | Canadian Athlete |
| Jose Ortega y Gasset | Spanish Philosopher |
| Daniel Ortega | Nicaraguan Statesman |
| David Ortiz | |
| Solomon Ortiz | American Politician |
| Beth Orton | English Musician |
| Joe Orton | English Playwright |
| George Orwell | British Author |
| Adam Osborne | |
| Buzz Osborne | American Musician |
| John Osborne | English Playwright |
| Tom Osborne | American Politician |
| Jack Osbourne | English Celebrity |
| Kelly Osbourne | English Actress |
| Ozzy Osbourne | English Musician |
| Sharon Osbourne | English Entertainer |
| Osceola | Seminole Statesman |
| Doug Ose | American Politician |
| Charles Osgood | American Journalist |
| William Osler | Canadian Scientist |
| Donny Osmond | American Musician |
| Marie Osmond | American Musician |
| Henry Ossawa Tanner | American Artist |
| Joel Osteen | American Clergyman |
| Wilhelm Ostwald | German Scientist |
| Marina Oswald | Russian Celebrity |
| Patton Oswalt | American Comedian |
| Cornelia Otis Skinner | American Actress |
| Carre Otis | American Model |
| James Otis | American Lawyer |
| Johnny Otis | American Musician |
| Mel Ott | American Athlete |
| Jim Otto | American Athlete |
| John Otto | |
| Miranda Otto | Australian Actress |
| Rudolf Otto | German Theologian |
| Thomas Otway | English Dramatist |
| Ouida | English Novelist |
| Thomas Overbury | English Poet |
| Ovid | |
| Michael Ovitz | American Businessman |
| Michael Owen Bruce | American Musician |
| Richard Owen Cambridge | British Poet |
| Clive Owen | English Actor |
| John Owen | English Theologian |
| Michael J. Owen | English Athlete |
| Richard Owen | English Scientist |
| Robert Owen | Welsh Writer |
| Wilfred Owen | English Soldier |
| Bill Owens | American Politician |
| Buck Owens | American Musician |
| Gary Owens | American Actor |
| Jesse Owens | American Athlete |
| Major Owens | American Politician |
| Major R. Owens | American Politician |
| Robert D. Owens | |
| Terrell Owens | American Athlete |
| John Oxenham | British Journalist |
| Michael Oxley | American Politician |
| Amos Oz | Israeli Writer |
| Frank Oz | American Actor |
| Cynthia Ozick | American Novelist |