The pural opi for opus is a joke, right?

Yes, it is an in-joke, which I can confirm as a former instrument player in orchestras.

It's not confined to the arts world though. I have heard people jokingly pluralise other words with -i that don't even end with -us. It's probably more common among those who learnt Latin at school (as I did).

Allied is the use of 'platypi' colloquially but in this case the word platypus was neologized from Greek roots.

(Dredging up my schoolgirl Latin) Many Latin nouns ending in -us are second declension, with plural -i. However, opus happens to be a third declension noun, and its plural is opera. See latindictionary.wikidot.com/noun:opus

As others have pointed out, octopus and platypus are derived from Greek, so they don't have plurals in -i either.