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.

I stand corrected. I should've known. Some neuter plurals in Greek are treated as singular in form. There's still no plural in Latin for virus 'poison' or pelagus 'sea'.

Ceci n'est pas un seing.