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- Kevin O'Connell (England, Ireland, born 1949)
- Handszar Odeev (Turkmenistan, born 1972)
- Leif Øgaard (Norway, born 1952)
- John O'Hanlon (Ireland, 1876–1960)
- Tõnu Õim (Estonia, born 1941)
- Kaarle Ojanen (Finland, born 1918)
- Albéric O'Kelly de Galway (Belgium, 1911–1980)
- Friðrik Ólafsson (Iceland, born 1935)
- Helgi Ólafsson (Iceland, born 1956)
- Mikhailo Oleksienko (Ukraine, born 1986)
- Lembit Oll (Estonia, 1966–1999)
- Adolf Georg Olland (Netherlands, 1867–1933)
- Anton Olson (Sweden, 1881–after 1928)
- Alexander Onischuk (Ukraine, USA born 1975)
- Karel Opočensky (Czechoslovakia, 1892–1975)
- Wilhelm Orbach (Germany, 1894–1944)
- Menachem Oren (Poland, Israel, 1901–1962)
- Gerard Oskam (Netherlands, 1880–1952)
- Berge Ostenstad (Norway, born 1964)
- John Owen (England, 1827–1901)
- Karlis Ozols (Latvia, Australia, 1912–2001)
Read more about this topic: List Of Chess Players