44 selected
- Not filled in
- Octavianus
- Odd Blomdal van Spitsbergen (1991-1995)
- Odoardo Farnese of Parma (1622-1646)
- Odumegwu, Ojukwu van Biafra (1967-1970)
- Olav V of Norway (1957-1991)
- Oleksandr Turchynov of Ukraine (2014)
- Ollanta Humala Tasso of Peru (2011-2016)
- Olusegun Obasanjo van Nigeria (1976-1979, 1999-2007)
- Omar al-Bashir of Sudan (1989-2019)
- Omar Ali Saifuddin III of Brunei (1950-1967)
- Omar Bongo Ondimba van Gabon (1967-2009)
- Omer Nishani van Albanië (1944-1953)
- Ong Teng Cheong van Singapore (1993-1999)
- Onofrio Fattori van San Marino (apr 1898-sept 1898, apr 1902-sept 1902, okt 1905-mrt 1906, okt 1911-mrt 1912, apr 1916-sept 1916, okt 1922-mrt 1923)
- Orodes I of the Parthian Empire (89-78 B.C.)
- Orodoes I of the Parthian Empire (80-75 BCE)
- Orodoes III of the Parthian Empire (4-6 CE)
- Oscar Humberto Mejia Victores van Guatemala (1983-1986)
- Oscar I of Sweden (1844-1859)
- Oscar II of Sweden (1872-1907) and Norway (1872-1905)
- Oscar Luigi Scalfaro van Italië (1992-1999)
- Oscar Mina van San Marino (Apr 2009-Sept 2009, Apr 2022-Sept 2022)
- Oscar Osorio van El Salvador (1950-1956)
- Oshin of the Cilician Kingdom of Armenia (1308-1320 AD)
- Osmín Aguirre y Salinas van El Salvador (1944-1945)
- Osvaldo Dorticós Torrado of Cuba (1959-1976)
- Osvaldo Hurtado Larrea of Ecuador (1981-1984)
- Oswaldo López Arellano of Honduras (1963-1971, 1972-1975)
- Otho van het Romeinse Rijk (69 AD)
- Ottaviano Rossi van San Marino (apr 1990-sept 1990)
- Otto Arosemena Gómez van Ecuador (1966-1968)
- Otto August Strandman of Estonia (1929-1931)
- Otto Fernando Pérez Molina van Guatemala (2012-2015)
- Otto I of Bavaria (1886-1913)
- Otto I of Greece (1832-1862)
- Otto III, Rooms-Duits koning (983–1002)
- Otto IV von Brandenburg (1266-1308)
- Otto Johannes Lundson van Finland (1917)
- Otto Stich of Switzerland (1988,1994)
- Otto von Lonsdorf van Passau (1254-1265)
- Ottokar II van Bohemen (1253-1278)
- Overijssel first stadtholderless era (1650-1675)
- Overijssel Second Stadtholderless Era (1702-1747)
- Oxatheres of Persis (50 BCE - 0 CE)