| Gattung | Titel | Komponist | Komponiert |
| Oper | Oberon | Carl Maria von Weber | 1826 |
| Oper | Oberto, comte de San Bonifacio | Giuseppe Verdi | 1839 |
| Oper | Oberto, Conte di San Bonifacio | Giuseppe Verdi | 1839 |
| Oper | Oberto, Count of San Bonifacio | Giuseppe Verdi | 1839 |
| Oper | Oberto, Graf von San Bonifacio | Giuseppe Verdi | 1839 |
| Oper | Obrutschenije w monastyre [Обручение в монастыре] | Sergei Sergejewitsch Prokofjew | 1940-41 |
| Oper | Oceana | Antonio Smareglia | 1903 |
| Oper | Ogier de Danemarche | Friedrich Ludwig Æmilius Kunzen | 1789 |
| Oper | Ogier the Dane | Friedrich Ludwig Æmilius Kunzen | 1789 |
| Oper | Olympia | Gaspare Luigi Pacifico Spontini | 1819, rev. 1821 und 1826 |
| Oper | Olympie | Gaspare Luigi Pacifico Spontini | 1819, rev. 1821 und 1826 |
| Oper | Opportunity Makes a Thief, or The Exchanged Suitcase | Gioacchino Rossini | 1812 |
| Oper | Oresteja [Орестея] | Sergei Iwanowitsch Tanejew | 1887-94 |
| Oper | Orfeo ed Euridice | Christoph Willibald Gluck | 1762, rev. 1774 |
| Oper | Orleanskaja dewa [Орлеанская дева] | Peter Iljitsch Tschaikowski | 1878-79 |
| Oper | Orontea | Pietro Antonio Cesti | 1656 |
| Operette | Orphée aux Enfers | Jacques Offenbach | 1858, rev. 1874 |
| Oper | Orphée et Euridice | Christoph Willibald Gluck | 1762, rev. 1774 |
| Oper | Orpheus and Eurydice | Christoph Willibald Gluck | 1762, rev. 1774 |
| Operette | Orpheus in der Unterwelt | Jacques Offenbach | 1858, rev. 1874 |
| Oper | Orpheus und Euridike | Christoph Willibald Gluck | 1762, rev. 1774 |
| Oper | Orseolo | Ildebrando Pizzetti | 1931-35 |
| Oper | Orséolo | Ildebrando Pizzetti | 1931-35 |
| Oper | Otello | Giuseppe Verdi | ? |
| Oper | Othello | Giuseppe Verdi | ? |