Not sure why all our bike sheds need to be the same color (is there really a need to standardize this?), but—since we’re heaving an exercise in C. Northcote Parkinson’s infamous law—
Personally, I’d just use “«Composer Name’s» Opus” and not bother with the plural. Opera would be fun to explain to people over and over again. Opuses just looks funny to me. Opi (using the octopus → octopi transform) would be silly, and that’s a silliness to which Wiktionary confirms English has not yet stooped.
I would strongly prefer to keep the composer’s name in there. Just “Works by opus number” make its too easy to select the wrong one (think of when you’re adding a few new works by a few composers, and you can currently use the completion history in the add relationship dialog—without the composer’s name, it’d be unusable.) I’d take “Opi” over that.
Maybe “«Composer Name’s» Numbered Works” Or “«Composer Name’s» Published Works”, if we really have to avoid the o-word . Or just “«Composer Name’s» Works" if we want it short.
Honestly, a lot of these generic catalogs actually have better names (with catalogers, no less).