Opus has a built-in tag editor which works well enough when you only need to edit one file at a time but, at the moment, Opus is not well suited to editing the tags in several files at the same time.
You can drop other music files on the tag editor to copy selected tags from them to the file being edited but it does not work the other way around and you are still only editing one file at a time.
Of course, this may be improved in a future version of Opus. For now I recommend using a dedicated tool for mass tag editing, such as:
[ul][li]The Godfather (TGF) is a free, dedicated tag editor. It’s very good and powerful, once you are over the short learning curve and get used to the user interface. The only real downside is that it does not support WMA files.
[li]Foobar2000 (FB2K) is a free music player which has an excellent mass-tag-editing feature. Unlike TGF it does support WMA files (and just about every other format). It’s also got a learning curve but is also very powerful. Perhaps not as nice as TGF for tagging but, since I use FB2K anyway for playing music and since much of my music is now WMA Lossless files, I now only use FB2K for tag editing. I’d probably dig out The Godfather if I had a lot of edits to do, though.
(Why WMA Lossless? Because just about everything, from iTunes to Nero’s command-line mode, can read the files, unlike other lossless formats.)
There are others, of course; those are just my personal top tips. I don’t just recommend them because they’re free; I think they are also the most powerful and, once you get used to them, convenient tools for the job, even if you consider the ones which cost money.