This does bring up an interesting topic. If the word has truly become an English word it should have an English style plural. The term opera, legitimate scholarly plural of opus notwithstanding, has been taken to mean something beyond its literal meaning of a collection of opuses, so to use it as the plain plural of opus would be misleading, particularly as the context is one of classical music. Opi is just plain wrong and shows an ignorance of the facts of the matter. Unless it was Peter Schickele, in which case he knew what he was doing and was just being Peter Schickele.

Opus is not so much an irregular noun as it is one with a different kind of regularity. The "regular" nouns of which TEd spoke are second declension and follow a paradigm with a root followed by a case ending. The root does not change from case to case. Nouns of the second declension ending in -us are masculine and those ending in -um are neuter. Opus is third declension. In the third declension all cases and number are formed of the root plus a case ending except nominative singular. In the nominatve singular the root is normally truncated or modified in some way. In the case of opus, the root is oper-. Also, the ending of the nominative singular does not indicate that the word is masculine. The fact that the nominative (and accusative) plural ends in -a does indicate that it is neuter.