Thank you for replying. The next morning when I started up Opera again, Duck Duck Go was the default search engine. Not sure what happened, as I had tried many things -- relaunching in safe mode, switching to Google (Opera's default) and then back to Duck Duck Go, closing Opera and re-opening, etc.... but it's all working fine now.

Only the pre-defined search engines that come with Opera can be set as the default one.

@leocg Evidently I broke a rule - sorry about that. I will read the instructions and repost appropriately. Thank you for your patience.

Google doesn't separate search results that much anymore - like they used to do for different language areas. Sure China still get's the treatment. If Opera continues to be the only browser with forced default engine ~ step 1 would add option to choose a country code (ccTLD) from a short list including .com!

@m-n-g I think it is only work in address bar not in Speed Dial bar. So, type in address bar

Edit :
Sorry, I didn't read this forum is for Linux.
I use Windows 10 v.1709 (RS3)

@silas229 Except, maybe, for Edge/IE and Chrome, almost all browsers get their revenues from searches and from pre-added bookmarks/speed dials.