Updating without user consent and without even informing the user afterwards is completely unacceptable and unheard of, so I don't think it works like that.
In fact I'm sure that a few versions ago I could go to the About Opera page, see the current version, be told if a new version was available, and click an "Update Opera" button if I wanted to update to that version.
So there is really no normal / "conformist" way to disable automatic updates, or at least to get Opera to ask for user consent ? It can only be done by hacking into some files / folders ?