I think what you’re describing is autocomplete, not the normal Autofill setting. Opera unfortunately doesn’t have one simple switch to turn off every type of address-bar autocomplete.
I’d try this first:
Settings → Privacy & security → disable “Improve search suggestions.”
You can also go to opera://settings/enhancedAddressBar and turn off the enhanced address bar suggestions.
If Opera is still filling in old sites from your history or bookmarks, those are handled separately, and there isn’t currently a complete “turn all autocomplete off” option. That’s probably why you’re still seeing the YouTube video suggestion.