Go into msconfig.exe and disable Opera in your startup processes. That’ll shut it up.
At least until the next update. And even then, it will sometimes magically turn itself back on. And if the Opera Assistant keeps getting turned back on, what else is there in the background with it? As I said, I don’t do critical account logins with the backup browser.
Funny thing. When I checked Windows 10's Task Manager a few days ago, I noted the "Last BIOS Time". It seemed a bit long, and it was one of the reasons I started looking at the startup process(es).
When I booted up this morning, this time was 11 seconds less than a couple days ago. The only thing I've done is uninstall Opera and install Vivaldi.
As some have indicated elsewhere, Opera is doing something in the background that is not readily apparent to the user. Consumer beware.