| Henry Steel Olcott | Union United States Army offi… | 1832–1907 |
| Mike Oldfield | English musician, multi-instr… | 1953 |
| Gary Oldman | English actor, screenwriter, … | 1958 |
| Sharon Olds | American poet | 1942 |
| Larisa Oleynik | actress | 1981 |
| Mark Oliphant | Governor of South Australia (… | 1901–2000 |
| Thomas Oliphant | | |
| Simone Bittencourt de Oliveira | Brazilian singer | 1949 |
| Jamie Oliver | British chef and media person… | 1975 |
| John Oliver | English comedian | 1977 |
| Mary Oliver | American writer | 1935–2019 |
| Revilo P. Oliver | American philologist | 1908–1994 |
| Robert T. Oliver | American academic | 1909–2000 |
| Laurence Olivier | British actor, director and p… | 1907–1989 |
| Ehud Olmert | Israeli politician, prime min… | 1945 |
| Edward James Olmos | American actor and director | 1947 |
| Robert Olmstead | American writer | 1954 |
| Thomas Olmsted | Roman Catholic prelate; 4th B… | 1947 |
| Martin Dada Abejide Olorunmolu | Nigerian catholic priest | 1948 |
| Ashley Olsen | actor from USA | 1986 |
| Elizabeth Olsen | American actress | 1989 |
| Gregg Olsen | American writer | 1959 |
| Johan Olsen | Norwegian political scientist | 1939 |
| Ken Olsen | American engineer and busines… | 1926–2011 |
| Charles Olson | American writer | 1910–1970 |
| Mancur Olson | American economist | 1932–1998 |
| Sigurd F. Olson | American conservationist | 1899–1982 |
| Ahmed Omaar | | 1987 |
| Bryan Lee O'Malley | Artist | 1979 |
| Mohammed Omar | Founder and former leader of … | 1959–2013 |
| Omarosa | American political aide and t… | 1974 |
| Stormie Omartian | American writer | 1942 |
| Omawumi | Nigerian musician and actress | 1982 |
| O. Timothy O'Meara | American mathematician | 1928–2018 |
| Stewart O'Nan | American writer | 1961 |
| Aristotle Onassis | Greek shipping magnate | 1906–1975 |
| Jacqueline Kennedy Onassis | public figure, First Lady to … | 1929–1994 |
| Michael Ondaatje | Canadian writer | 1943 |
| Patrice O'Neal | American stand-up comedian, r… | 1969–2011 |
| Shaquille O'Neal | American basketball player | 1972 |
| Bill O'Neill | Ohio judge | 1947 |
| Eugene O'Neill | American playwright, and Nobe… | 1888–1953 |
| Gerard O'Neill | Physicist, author, and invent… | 1927–1992 |
| Tip O'Neill | American politician | 1912–1994 |
| Silouan Oner | Antiochian Orthodox metropoli… | 1970 |
| L. Onerva | Finnish poet | 1882–1972 |
| Juan Carlos Onetti | Uruguayan novelist | 1909–1994 |
| Michel Onfray | French philosopher | 1959 |
| Honesto Ongtioco | Filipino bishop | 1948 |
| Yoko Ono | Japanese artist, author, and … | 1933 |
| Hiroo Onoda | Imperial Japanese Army intell… | 1922–2014 |
| Lupe Ontiveros | Mexican-American actress | 1942–2012 |
| Jan Oort | Dutch astronomer | 1900–1992 |
| Kenneth Oppel | Canadian children's writer | 1967 |
| George Oppen | American poet | 1908–1984 |
| Robert Oppenheimer | American theoretical physicis… | 1904–1967 |
| Egan O'Rahilly | Irish poet | 1670–1726 |
| Tommy Orange | writer | 1982 |
| Viktor Orbán | Hungarian politician, chairma… | 1963 |
| Robert Orben | American magician and writer | 1928 |
| Emma Orczy | Hungarian-British author of "… | 1865–1947 |
| Grigoriy Ordzhonikidze | Soviet politician | 1886–1937 |
| Bill O'Reilly | American political commentato… | 1949 |
| John Boyle O'Reilly | Irish-born poet and novelist | 1844–1890 |
| Tim O'Reilly | Irish computer programmer | 1954 |
| Marcus Orelias | American actor, rapper, songw… | 1993 |
| Melissa Lackey Oremus | American politician | 1978 |
| Peggy Orenstein | American writer | 1961 |
| Nicole Oresme | French philosopher | 1323–1382 |
| Carl Orff | German composer | 1895–1982 |
| Elliott Organick | American computer scientist | 1925–1985 |
| Origen | Christian scholar in Alexandr… | 185–254 |
| Dolores O'Riordan | Irish singer | 1971–2018 |
| Dael Orlandersmith | American actress and writer | 1959 |
| Susan Orlean | American journalist | 1955 |
| Wanda Orlikowski | American computer scientist | |
| Suze Orman | American author, television p… | 1951 |
| Paul Ormerod | English economist | |
| Lauren Ornelas | American activist | |
| Dean Ornish | American physician | 1953 |
| Beto O'Rourke | American politician | 1972 |
| Colm O'Rourke | Irish Gaelic football player,… | 1957 |
| Meghan O'Rourke | American writer | 1976 |
| P. J. O'Rourke | American journalist | 1947 |
| David Orrell | Canadian mathematician | 1962 |
| Brig. Gen. Eran Ortal | | 1971 |
| Daniel Ortega | President of Nicaragua | 1945 |
| Oscar Urbina Ortega | Catholic archbishop | 1947 |
| David Ortiz | Dominican-American profession… | 1975 |
| Joe Orton | English playwright and author | 1933–1967 |
| George Orwell | English author and journalist | 1903–1950 |
| Eric Osborn | | 1922–2007 |
| Henry Fairfield Osborn | American geologist, paleontol… | 1857–1935 |
| John Osborne | English playwright | 1929–1994 |
| Mary Pope Osborne | American children's writer | 1949 |
| Jack Osbourne | Son of Ozzy and Sharon Osbour… | 1985 |
| Kelly Osbourne | English singer-songwriter, ac… | 1984 |
| Ozzy Osbourne | English heavy metal vocalist … | 1948 |
| Arthur O'Shaughnessy | British poet | 1844–1881 |
| Hitoshi Oshitani | Japanese physician | 1959 |
