Good work. If you do not have chromium installed too, pin it so that apt won't upgrade it to the version from the repo
sudo apt-mark hold chromium-codecs-ffmpeg-extra
And npin it when opera stable reaches 62 like so
sudo apt-mark unhold chromium-codecs-ffmpeg-extra
It should only need to ask once per session. That is to say, I use both Opera and Vivaldi, between them I only get asked once (by whichever runs first) until I log out of my desktop. If I close Opera and do something else for a while then go back to Opera it doesn't ask me then either (unless I logged out of the desktop)..
Now, if it asks but you somehow lose the dialog (behind another window, or close it by accident or some such) then Opera won't know any passwords ... but it also won't lose them. If you close Opera then try again all the old passwords will still be there, as it couldn't save them because you weren't logged in to the password manager.
I presume there must be a way to reset the password manager, but I'm also sure you'd know if you had..