![p](templates/images/picture.png) | Seán Ó Riada | 1931 - 1971 | Ireland | 1 |
| Anton Ulrich Obermayr | 18th - century | Austria | 2 |
| Johann Obersteiner | 1824 - 1896 | Austria | 7 |
![p](templates/images/picture.png) | Jacob Obrecht | 1457/58 - 1505 | The Netherlands / Belgium | 3 |
![p](templates/images/picture.png) | Oscar OBrien | 1892 - 1958 | Canada | 1 |
![p](templates/images/picture.png) | Fergus OCarroll | 1956 - | Ireland | 1 |
| Tomás Ochando | c.1725 - c.1799 | Mexico | 3 |
| Traugott Ochs | 1854 - 1919 | Germany | 1 |
![p](templates/images/picture.png) | Johannes Ockeghem | c.1420 - 1497 | The Netherlands / Belgium | 2 |
![p](templates/images/picture.png) | Tomás ODrisceoil | 1986 - | Ireland | 1 |
![p](templates/images/picture.png) | Helmut Oehring | 1961 - | Germany | 1 |
![p](templates/images/picture.png) | Johann Lohelius Oelschlegel | 1724 - 1788 | Czech Republic | 1 |
![p](templates/images/picture.png) | Bart Oenema | 1967 - | The Netherlands | 1 |
![p](templates/images/picture.png) | Jan van Oers | 1860 - 1941 | The Netherlands | 1 |
![p](templates/images/picture.png) | Rudolf von Oertzen | 1910 - 1990 | Germany | 1 |
![p](templates/images/picture.png) | Seung-Won Oh | 1969 - | The Netherlands | South Korea | 1 |
| Joseph Ohnewald | 1781 - 1856 | Germany | 3 |
![p](templates/images/picture.png) | Hilario Olazaran de Estella | 1894 - 1963 | Spain | 2 |
![p](templates/images/picture.png) | Juan Manuel Olivares | 1760 - 1797 | Venezuela | 4 |
![p](templates/images/picture.png) | Domingo Olleta Mombiela | 1819 - 1890 | Spain | 1 |
![p](templates/images/picture.png) | Otto Emanuel Olsson | 1879 - 1964 | Sweden | 1 |
![p](templates/images/picture.png) | Roel van Oosten | 1958 - | The Netherlands | 1 |
![p](templates/images/picture.png) | Wytze Oostenbrug | 1943 - | The Netherlands | 1 |
![p](templates/images/picture.png) | György Orbán | 1947 - | Rumania | Hungary | 2 |
![p](templates/images/picture.png) | Conor OReilly | 1967 - | Ireland | 1 |
![p](templates/images/picture.png) | Joaquin Orellana Mejia | 1937 - | Guatemala | 1 |
![p](templates/images/picture.png) | Carl Orff | 1895 - 1982 | Germany | 1 |
| Vincenzo Orgitano | c.1738 - aft.1815 | Italy | 1 |
![p](templates/images/picture.png) | Charles Orr | 1893 - 1976 | Great Britain, England | 1 |
![p](templates/images/picture.png) | Juan Orrego Salas | 1919 - 2019 | Chile | United States of America | 1 |
![p](templates/images/picture.png) | Eduard Ortlieb | 1807 - 1861 | Germany | 1 |
| Antonius Ortner | fl. 1827 - 1858 | Austria | 1 |
| Clara Osmond | fl.1940 - | Great Britain, England ? | 1 |
![p](templates/images/picture.png) | Helen Ostafew | 20th - century | Great Britain, England | 1 |
![p](templates/images/picture.png) | Henrique Oswald | 1852 - 1931 | Brazil | 1 |
![p](templates/images/picture.png) | Nemesio Otańo y Eguino | 1880 - 1956 | Spain | 1 |
![p](templates/images/picture.png) | Caspar Othmayr | 1515 - 1553 | Germany | 2 |
| Stanislav Otruba | 1868 - 1949 | Czech Republic | 1 |
| Lorenz Justinian Ott | 1748 - 1805 | Germany | 1 |
![p](templates/images/picture.png) | Matt Ottley | 1962 - | Papua New Guinea | Australia | 1 |
![p](templates/images/picture.png) | Julius Otto | 1804 - 1877 | Germany | 1 |