@david27 Spotify Web doesn't seem to have such setting.
So it depends on Spotify's web app, not Opera. I never use Spotify on the web, I even forgot it exists.
I've just made a search and found this:
"From the web app, well, you don’t have much of a choice on sound quality. If you have Spotify Premium, music will play back at 256kbps. If you have free Spotify, it’ll play at 128kbps."
Too bad it isn't 320kbps like in the desktop app, but 256kbs is decent enough.