Composers – O
- O'Boyle, Sean (b.1963)
- O'Carolan, Turlough (1670–1738)
- O'Connor, Frederick (1870-1943)
- O'Connor, Mark (b.1961)
- O'Halloran, Dustin (b.1971)
- O'Hara, Geoffrey (1882–1967)
- O'Neill, Michael (b.1955)
- O'Rourke, Aidan (b. 1975)
- Oakland, Ben (1907-79)
- Oberfeld, Casimir (1903-45)
- Obradors, Fernando Jaumandreu (1897–1945)
- Ochs, Larry (b.1949)
- Oda, Kazumasa (b. 1947)
- Offenbach, Jacques (1819–80)
- Ogermann, Claus (1930–2016)
- Ogiński, Michał Kleofas (1765-1833)
- Okano, Tei'ichi (1878–1941)
- Olah, Kálmán (b.1970)
- Oldfield, Mike (b.1953)
- Olias, Lotar (1913-90)
- Oliveira, João Pedro (b.1959)
- Oliver
- Oliver, Stephen (1950–92)
- Olivieri, Dino (1905–63)
- Oliviero, Nino (1918-80)
- Olman, Abe (1887-1984)
- Olofsson, Olov
- Olrog, Ulf Peder (1919–72)
- Olshanetsky, Alexander (1891–1946)
- Olsson, Otto Emanuel (1879–1964)
- Ono, Yoko (b.1933)
- Oppenheim, Henri
- Orbán, György (b.1947)
- Ord, Boris (1897-1961)
- Orff, Carl (1895–1982)
- Orlando, Stéphane
- Orlova, Natalia
- Ortiz, Carlos Vieco (1900-79)
- Ortiz, Diego (1510–70)
- Orton, Richard (1940-2013)
- Osborne, Tony (b.1947)
- Ostrowski, Tomasz (b.1975)
- Oteo, Alfonso Esparza (1894–1950)
- Otero, Fernando (b. 1972)
- Otte, Hans (1926–2007)
- Otterloo, Rogier van (1941-88)
- Ottman, John (b.1964)
- Otto, Michael T (b.1969)
- Ourkouzounov, Atanas (b.1970)
- Ovalle, Jaime (1894–1955)
- Overton, Hall (1920-72)
- Owens, Harry (1902-86)
- Oxley, Harrison (1933-2009)
- Ozone, Makoto (b. 1961)
