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- Jacob Obrecht (c. 1457/1458–1505)
- Johannes Ockeghem (c. 1410/1425–1497)
- Martin O'Donnell (born 1955)
- Jacques Offenbach (1819–1880)
- Will Ogdon (born 1921)
- Tale Ognenovski (born 1922)
- Maurice Ohana (1913–1992)
- Mike Oldfield (born 1953)
- Carolina Oliphant (1766–1845)
- Pauline Oliveros (born 1932)
- Otto Olsson (1879–1964)
- George Onslow (1784–1853)
- Cornélie van Oosterzee (1863–1943)
- Chris Opperman (born 1978)
- Mathilda d'Orozco (1796–1863)
- Julián Orbón (1925–1991)
- Karl von Ordoñez (1734–1786)
- Cecilie Ore (born 1954)
- Carl Orff (1895–1982)
- Giuseppe Maria Orlandini (1676–1760)
- Leo Ornstein (1893–2002)
- Aniceto Ortega (1825–1875)
- Marbrianus de Orto (c. 1460–1529)
- Riz Ortolani (born 1931)
- Alex Otterlei (born 1968)
- Georg Österreich (1664–1735)
- Willy Ostijn (1913–1993)
- Henrique Oswald (1852–1931)
- James Oswald (1711–1769)
- Hans Otte (1926–2007)
- Willem van Otterloo (1907–1978)
- Frederick Ouseley (1825–1889)
- Morfydd Owen (1891–1918)
- Etienne Ozi (1754–1813)
Read more about this topic: List Of Composers By Name