Opera is also the plural of opus in Latin. This meaning has become archaic, as it would lead to confusion with the aforementioned art form, but it's an interesting bit of music trivia, and might help you with a crossword puzzle some day. In English the usual plural of 'opus' is 'opuses', and in Italian 'opere', since 'opera' has become a singular noun there as well.

So now if you've written a few songs you can tell people you've "composed several opera."