re latin plurals/singulars.
Faldage points out words of which we use the plural more often than the singular. Datum and medium are used in UK as singular forms of data and media (though more rarely now) but I haven't heard agendum.
I don't think I have heard opera as plural of opus, writers tend to use the word opus** as a collective noun and avoid the problem. POD lists opera as a singular noun for a musical work.

Rod
or should we refer to the recent definitive discussion on "octopus" in this board?