| Name | Dates | Era | Nationality |
|---|---|---|---|
| Obrecht, Jacob | c. 1450 - 1505 | Renaissance | Netherlands |
| Ockeghem, Johannes | c. 1420 - 1497 | Renaissance | Netherlands |
| Offenbach, Jacques | 1819 - 1880 | Romantic | France |
| Orff, Carl | 1895 - 1982 | Romantic | Germany |
| Orrego-Salas, Juan Antonio | 01/18/1919 - 11/24/2019 | 20th Century | Chile |
| Osmon, Leroy | 05/27/1948 | 20th Century | USA |