I found this with an internet search. I could find nothing like this within Opera itself. I must have missed it.
Go to the address bar and enter: "opera://flags" (without quotes)
That will take you to a long list of options, one of which is to enable smooth scrolling.

Why smooth scrolling is not enabled out of the box and why it is hidden in opera://flags (which doesn't seem to be announced anywhere obvious) is a complete mystery.

Manjaro Linux with Budgie DE (both up to date)
Dell Inspiron Laptop 7300 Series

I installed opera-stable_68.0.3618.125_amd64 today and I'm experiencing scrolling problems, which appear to be somewhat different on different sites.
On this site, the scrolling is jerky and sometimes stalls momentarily with the cursor inoperative: www.cbc.ca/news
On various other sites there is a slight jerky "fluttering" effect to the scrolling.
Suggested remedies appreciated.