| Carolina la O | Colombian musician | 1975 |
| Paul Oakenfold | British record producer and a… | 1963 |
| Fiona Oakes | British marathon runner | 1966 |
| Michael Oakeshott | British philosopher | 1901–1990 |
| Philip Oakey | English pop singer | 1955 |
| Dallin H. Oaks | Apostle of the LDs Church | 1932 |
| Joyce Carol Oates | American author | 1938 |
| Lawrence Oates | British explorer | 1880–1912 |
| Keith Oatley | Anglo-Canadian psychologist | 1939 |
| Barack Obama | 44th President of the United … | 1961 |
| Michelle Obama | lawyer, writer, wife of Barac… | 1964 |
| James O'Barr | Artist | 1960 |
| Conor Oberst | American musician | 1980 |
| Chigozie Obioma | Nigerian writer | 1986 |
| Alanis Obomsawin | Abenaki filmmaker in Montreal | 1932 |
| Andrés Manuel López Obrador | president of Mexico | 1953 |
| Patrick O'Brian | English novelist | 1914–2000 |
| Conan O'Brien | American television show host… | 1963 |
| Conor Cruise O'Brien | Irish politician | 1917–2008 |
| Edna O'Brien | Novelist, memoirist, biograph… | 1930 |
| Flann O'Brien | Irish writer | 1911–1966 |
| Tim O'Brien | American novelist | 1946 |
| Walter O'Brien | Irish businessman, founder of… | 1975 |
| William Smith O'Brien | Irish nationalist politician … | 1803–1864 |
| Ambeth R. Ocampo | Filipino historian | 1961 |
| Sean O`Casey | Irish writer | 1880–1964 |
| Alexandria Ocasio-Cortez | American politician | 1989 |
| Thomas Occleve | British writer | 1369–1426 |
| Frank Ocean | American musician | 1987 |
| Phil Ochs | American protest singer and s… | 1940–1976 |
| Robyn Ochs | American activist | 1958 |
| William of Ockham | medieval philosopher and theo… | 1285–1349 |
| Daniel O'Connell | Irish political leader | 1775–1847 |
| Mary O'Connor | | |
| Peg O'Connor | American philosopher | 1965 |
| Sandra Day O'Connor | Former Associate Justice of t… | 1930 |
| Sean O'Connor | | |
| Joseph O`Connor | Anglo-Irish actor and playwri… | 1916–2001 |
| Flannery O’Connor | American novelist, short stor… | 1925–1964 |
| Hotaru Odagiri | Japanese mangaka and illustra… | |
| John Baptist Odama | born 1947; Roman-Catholic Arc… | 1947 |
| Ayman Odeh | Israeli lawyer and member of … | 1975 |
| Tom Odell | British singer-songwriter | 1990 |
| Scott O'Dell | American children's writer | 1898–1989 |
| Tawni O'Dell | American writer | 1964 |
| Clifford Odets | Playwright, screenwriter, dir… | 1906–1963 |
| Christine O'Donnell | American Tea Party politician… | 1969 |
| Cyril J. O'Donnell | American professor | 1900–1976 |
| Rosie O'Donnell | American comedienne, televisi… | 1962 |
| Michael O'Donoghue | American actor and writer | 1940–1994 |
| John O'Donohue | Irish writer, priest and phil… | 1956–2008 |
| Chris O'Dowd | Irish actor and comedian (bor… | 1979 |
| Brian O'Driscoll | | |
| Dennis O'Driscoll | Irish poet, critic | 1954–2012 |
| Eugene P. Odum | mathematician, ecologist, nat… | 1913–2002 |
| Roger von Oech | Creativity theorist and inven… | 1948 |
| Nuala O'Faolain | Irish writer | 1940–2008 |
| Maggie O'Farrell | British writer | 1972 |
| Robert Anderson (Scotland Yard official) | | |
| Tom Dooley (American football official) | | |
| Jenny Offill | American writer | 1968 |
| Paul Offit | American immunologist | 1951 |
| Liam O'Flaherty | Irish writer | 1896–1984 |
| Yōko Ogawa | Japanese writer | 1962 |
| Charlton Ogburn | American journalist and author | 1911–1998 |
| John Ogilby | Scottish academic | 1600–1676 |
| David Ogilvy | Advertising executive | 1911–1999 |
| Carl Oglesby | American political activist | 1935–2011 |
| Madalyn Murray O'Hair | Atheist activist | 1919–1995 |
| Frank O'Hara | American poet, art critic and… | 1926–1966 |
| John O'Hara | American journalist | 1905–1970 |
| Maureen O'Hara | Irish-American film actress a… | 1920–2015 |
| Robert O'Hara | American playwright and theat… | |
| Otto Ohlendorf | German general | 1907–1951 |
| Bertil Ohlin | Swedish economist and politic… | 1899–1979 |
| Georg Ohm | German physicist and mathemat… | 1789–1854 |
| Kenichi Ohmae | Japanese academic | 1943 |
| Apolo Anton Ohno | American short track speed sk… | 1982 |
| Taiichi Ohno | Japanese businessman and engi… | 1912–1990 |
| Haley O'Hora | | |
| George Ohsawa | twentieth century Japanese ph… | 1893–1966 |
| Naoto Ohshima | Japanese video game artist | 1964 |
| David Oistrakh | Soviet violinist | 1908–1974 |
| Chukwuemeka Odumegwu Ojukwu | Nigerian politician and milit… | 1933–2011 |
| Chuba Okadigbo | Nigerian politician | 1941–2003 |
| Kakuzo Okakura | Japanese scholar, author of T… | 1862–1913 |
| Yasuji Okamura | Japanese general | 1884–1966 |
| Willa Okati | | |
| Ritsuko Okazaki | Japanese singer | 1959–2004 |
| Janette Oke | Canadian writer | 1935 |
| James O'Keefe | American conservative filmmak… | 1984 |
| Georgia O'Keeffe | American artist | 1887–1986 |
| Pat O'Keeffe | British boxer | 1883–1960 |
| Nobuo Okishio | Japanese economist | 1927–2003 |
| Nnedi Okorafor | Nigerian-American writer of f… | 1974 |
| Ben Okri | Nigerian writer | 1959 |
| Michael Okuda | graphic designer known for wo… | |
| Hakeem Olajuwon | Nigerian–American basketball … | 1963 |
| Keith Olbermann | American sports and political… | 1959 |
| Heinrich Wilhelm Matthäus Olbers | German physician and astronom… | 1758–1840 |
