- O'Brien, Matthew (Ireland, 1814–1855)
- O'Connell, Neil (Ireland, ?–?)
- O'Dorney, Evan (USA, born 1993)
- O'Hara, Jun (Japan, born 1963)
- O'Keefe, Christine (Australia, ?–?)
- O'Leary, Dianne P. (?, born 1951)
- O'Malley, Robert Edmund (USA, born 1939)
- O'Meara, O. Timothy (USA, 1928–2018)
- O'Nan, Michael (USA, 1943–2017)
- O'Neill, Barrett (USA, 1924–2011)
- O'Shea, Donal (Canada, ?–?)
- Obádovics, Gyula J. (Hungary, born 1927)
- Oberschelp, Arnold (Germany, born 1932)
- Obi, Chike (Nigeria, 1921–2008)
- Obreshkov, Nikola (Bulgaria, 1896–1963)
- d'Ocagne, Philbert Maurice (France, 1862–1938)
- Ockendon, Hilary (Britain, ?–?)
- Ockendon, John (Britain, born 1940s)
- Ocone, Daniel (USA, ?–?)
- Oda, Tadao (Japan, born 1940)
- Oddi, Muzio (Italy, 1569–1639)
- Odlyzko, Andrew (USA, born 1949)
- Oellermann, Ortrud (Canada/South Africa, ?–?)
- Oenopides (Ancient Greece, born 490s BC)
- Oesterlé, Joseph (France, born 1954)
- Offord, Cyril (?, 1906–2000)
- Ofman, Yuri Petrovich (Russia, born 1939)
- Ogden, Raymond (Britain, ?–?)
- Ogg, Andrew (USA, born 1934)
- Oggier, Frédérique (Switzerland/Singapore, ?–?)
- Ogilvy, C. Stanley (USA, 1913–2000)
- Ognjanović, Srđan (Serbia, ?–?)
- Ogura, Kinnosuke (Japan, ?–?)
- Ogus, Arthur (USA, ?–?)
- Oh, Hee (South Korea, born 1969)
- Oh, Yong-Geun (South Korea, born 1961)
- Ohm, Martin (Germany, 1792–1872)
- Ohya, Masanori (Japan, born 1947)
- Oja, Eve (Estonia, ?–?)
- Oka, Kiyoshi (Japan, 1901–1978)
- Okamoto, Hisashi (?, born 1956)
- Okamura, Hiroshi (Japan, 1905–1948)
- Okikiolu, Kathleen Adebola (Britain, born 1965)
- Okounkov, Andrei (Russia, born 1969)
- Øksendal, Bernt (Norway, born 1945)
- Oldroyd, James G. (Britain, 1921–1982)
- Olds, Carl D. (New Zealand, 1912–1979)
- Olearius, Adam (Germany, 1603–1671)
- Olech, Czesław (Poland, 1931–2015)
- Oleinik, Olga (Soviet Union, 1925–2001)
- Olesen, Dorte (Denmark, born 1948)
- Olevskii, Alexander Moiseevich (Soviet Union/Israel, born 1939)
- Olive, David (?, 1937–2012)
- Olive, Gloria (New Zealand, 1923–2006)
- de Oliveira, Adriana Neumann (Brazil, ?–?)
- Olivier, Théodore (?, 1793–1853)
- Oliynyk, Todd (Australia, ?–?)
- Olkin, Ingram (USA, 1924–2016)
- Ollerenshaw, Kathleen (Britain, 1912–2014)
- Olney, Grant (USA, born 1983)
- Olshansky, Aleksandr (Soviet Union/Russia, born 1946)
- Olubummo, Adegoke (Nigeria, ?–?)
- Olubummo, Yewande (Nigeria, ?–?)
- Olum, Paul (USA, 1918–2001)
- Olunloyo, Victor Omololu (Nigeria, born 1935)
- Olver, Frank W. J. (USA, 1924–2013)
- Olver, Peter J. (USA, born 1952)
- Onicescu, Octav (Romania, 1892–1983)
- Onishchik, Arkady Lvovich (Russia, 1933–2019)
- Ono, Ken (USA, born 1968)
- Ono, Takashi (?, born 1928)
- Onumanyi, Peter (Nigeria, ?–?)
- Onuş, Melih (Turkey, ?–?)
- Oommen, John (India, born 1953)
- Oort, Frans (?, born 1935)
- Opelt, Friedrich Wilhelm (Germany, 1794–1863)
- Oppenheim, Alexander (Britain, 1903–1997)
- Oppermann, Ludvig (Denmark, 1817–1883)
- Oppitz, Volker (Germany, born 1931)
- von Oppolzer, Theodor (Austria, 1841–1886)
- Ore, Øystein (Norway, 1899–1968)
- Oren, Menachem (Israel, 1903–1962)
- Oresme, Nicole (Medieval France, 1320s–1382)
- Orlando, Luciano (Italy, 1887–1915)
- Orlicz, Władysław (Poland, 1903–1990)
- Orlik, Peter (?, born 1938)
- Orlov, Dmitri Olegovich (Soviet Union, born 1966)
- Orno, Peter (USA, ?–?)
- Ornstein, Donald Samuel (USA, born 1934)
- Ornstein, Leonard (?, 1880–1941)
- Orr, William McFadden (Ireland, 1866–1934)
- Orrell, David (Canada, born 1962)
- Orshansky, Mollie (?, 1915–2006)
- Orszag, Steven (USA, 1943–2011)
- de Ortega, Juan (mathematician) (Spain, ?–?)
- Ortlieb, Claus Peter (?, 1947–2019)
- Osborn, John E. (USA, 1936–2011)
- Osgood, William Fogg (USA, 1864–1943)
- Osher, Stanley (USA, born 1942)
- Osin, Denis (?, ?–?)
- Osinga, Hinke (Netherlands/New Zealand, born 1969)
- Osipov, Yury (Russia/Soviet Union, born 1936)
- Osipovsky, Timofei (Russia, 1766–1832)
- Osofsky, Barbara L. (?, ?–?)
- Osserman, Robert (USA, 1926–2011)
- Ossiander, Mina (?, ?–?)
- Ossona de Mendez, Patrice (France, born 1966)
- Osterwalder, Konrad (Switzerland, born 1942)
- Osthus, Deryk (?, ?–?)
- Ostrogradsky, Mikhail (Ukraine/Imperial Russia, 1801–1862)
- Ostrovskii, Iossif (Soviet Union, 1934–2020)
- Ostrowski, Alexander (Ukraine, 1893–1986)
- Otho, Valentinus (Germany, ?–1603)
- Ott, Edward (?, ?–?)
- D'Ottaviano, Itala (Brazil, born 1944)
- Ottlik, Géza (Hungary, 1912–1990)
- Otto, Felix (Germany, born 1966)
- van Oudenaarden, Alexander (?, born 1970)
- Ouedraogo, Marie Françoise (?, born 1967)
- Oughtred, William (England, 1575–1660)
- Oum, Sang-il (South Korea, born 1976)
- Overbeek, Ross (USA, born 1949)
- D'Ovidio, Enrico (?, ?–?)
- Owen, Guillermo (Colombia, born 1938)
- Owen, Henry (Wales, 1716–1795)
- Owens, Helen Brewster (?, 1881–1968)
- Owens, Robyn (Australia, ?–?)
- Oxley, James (Australia, ?–?)
- Oxtoby, John C. (?, 1910–1991)
- Oystaeyen, Fred Van (Belgium, born 1947)
- Ozanam, Jacques (France, 1640–1718)
- Ozawa, Narutaka (?, born 1974)
- Ozsváth, Peter (Hungary, born 1967)
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