I've turned off some of the "cruft" they add to the browser, some related to advertising. Turned off "Instant Search", "My Flow", "Crypto Wallet", "Personal News", "Receive prompted Speed Dials and Bookmarks", "Show Speed Dial suggestions", "Show news on Start Page", "Show Continued Shopping section", "Show Continue on Booking.com section", "Show tab previews", all those are found under the "Advanced" section in "Settings".
I use a few extensions to add features, some from the Opera extensions site and a few from the Chrome extension site, since both browsers are based on Chromium. But be sure to check those from the official Chrome store out thoroughly though, because Google doesn't screen them as closely as they should, and some could be spyware/malware. The ones I currently use are listed below...
1. Ublock Origin : The best of all the ad blockers IMO. I never blocked ads until around four years ago, when a rogue banner ad tried to throw malware at me. Luckily my antivirus caught it before any harm was done.
3. User-Agent Switcher : Handy for spoofing the browser user agent string for websites that are looking for a more "major" browser.
4. Webcompat.com Reporter : Handy for reporting websites who aren't using standards and only want you to use one of the few "major" browsers.
5. SimpleTabOrder : Adds functionality to the way tabs work, better than Opera's built in system.
6. Mute Tab : Extension to keep websites that might have autoplaying audio quiet until you are ready to hear them.
7. Adblock for Youtube : Adblock for Youtube is a content filtering and ad blocking browser extension which blocks all ads on Youtube.
8. Install Chrome Extensions : Allows you to install extensions from the Chrome Web Store into Opera, for the ones not on the Opera extension website. Most work fine in Opera.
9. History Disabler : Allows you to keep all browsing history, including downloads, automatically deleted.
10. Download History Cleaner (Eraser) : The easiest way to clear your browser download history for a specified period. You can define a custom period on the options page.
11. Forecastfox (fix version) : Handy weather extension for the URL bar.
12. Cookie AutoDelete : Adds cookie control functionality that isn't built in to any browser that I'm aware of.
13. AutoplayStopper : Best extension I've found so far to stop pesky autoplaying videos on websites.
14. Webmail Ad Blocker : Block ads on the right side of the screen when using Gmail, Yahoo Mail, Hotmail and Outlook.com which expand your message space.
15. Share on Twitter : Share current tab, links and selected text on Twitter.
16. Share to Facebook : Easily share any link to Facebook, Groups, and Messenger.