Did you just download it, and not install it? Whether you installed it or not, the original downloaded Opera file should be in your Downloads folder (if that is the one you designated in your browser). If it's not there, you can do a search for Opera. If you did install Opera, the first thing you can do is download and install the excellent freeware program AppCleaner, available from here:
https://freemacsoft.net/appcleaner/
It works very well, and finds most files associated with the application you want to remove (along, of course, with the application itself).
Once that is done, you can do a search for "Opera" on your drive to find any further occurrences of "stuff" associated with Opera.