From Snopes :

Yet another group of Latin nouns in -us follow different rules. If you're still taking notes, these form another subset of the third declension. Typically in these cases the singular does not include the full root. The plural of genus is genera (not genuses and certainly not geni). In English the plural of opus (meaning a creative work) is opera (or opuses). In Latin, opera was originally the plural of opus, but in both Latin and English, opera can correctly be treated as a singular. In English the plural of opera (the thing that ain't over till the fat lady sings) is operas. When opera is used as a singular in Latin (where it meant more of less the same thing as opus), the correct plural is operae, but this is rarely if ever used in English.

Just trying to make operae a little less <ahem> rare, fellas.