@leocg It wasn't. Opera wasn't even the forefront application when it happened. Slack was.
In my cookies, there was one for rktch.com, which is a site I never went to, so no idea how they set a cookie. Out of curiosity, I went there and saw this:
(I didn't further configure anything.)
Then I found this blurb at a tech site.
Ngnix is the main culprit responsible for random redirects and spontaneous opening of new tabs with the advertising content.
I also have a bunch of .ru cookies, which I deleted and added to my Block list in settings/content/cookies. No idea how I acquired those either.
I hope this fixes the problem and prevents it from happening again.