| Oba Chandler | 1989 - 1990 | USA | 4 |
| Odell Barnes Jr. | 1989 | USA | 1 |
| Olaf Däter | 2001 | Germany | 5 |
| Oleg Vladimirovich Kuznetsov | 1991 - 1992 | Russia / Ukraine | 10 |
| Oleg Yurievich Chizhov | 2006 - 2007 | Russia | 4 |
| Olga Hepnarová | 1973 | Czech Republic | 8 |
| Olga Konstantinovna Strukova Briscorn | 1818 - 1822 | Russia | 128 |
| Omaima Aree Nelson | 1991 | Egypt | 1 |
| Omar Mir Seddique Mateen | 2016 | USA | 49+ |
| Ondrej Rigo | 1990 - 1992 | Slovakia | 9 |
| One L. Goh | 2012 | South Korea (Naturalized U.S. citizen) | 7 |
| Orhan Aksoy | 2000 - 2001 | Turkey | 5 |
| Orville Lynn Majors | 1993 - 1995 | USA | 6+ |
| Oscar Franklin Smith | 1989 | USA | 3 |
| Oscar Joseph Comery | 1914 | Canada / USA | 1 |
| Oscar Leonard Carl Pistorius | 2013 | South Africa | 1 |
| Oscar Ray Bolin | 1986 - 1987 | USA | 4 |
| Otis Tyrone McKane | 2016 | USA | 1 |
| Otoya Yamaguchi | 1960 | Japan | 1 |
| Ottilie Klimek | 1914 - 1921 | USA | 5+ |
| Ottis Toole | 1961 - 1983 | USA | 65+ |
| Otto Adolf Eichmann | 1939 - 1945 | Germany / Austria | 5,000,000+ |
| Otto Johann Anton Skorzeny | 1943 - 1945 | Austria | 1+ |
| Özgür Dengiz | 2007 | Turkey | 3 |
| Özkan Zengin | 2008 | Turkey | 3 |
