I looked some more and found this by a dev(?):

"https://www.aeyoun.com/ Daniel Aleksandersen
This will be renabled in a future update. Currently disabled due to third-party applications overriding the user’s preference in favour of malicious sites."
http://blogs.opera.com/desktop/2013/12/opera-desktop-browser-initial-19-next-edition-19-0-1326-9/

That was posted 4 months ago.
The temporary "fix" doesn't really have power users in mind, does it?

I guess I'll wait.

Hello,

I'm trying to make DuckDuckGo a default search engine in Opera 20 and can't find how. I added it other search engines (opera://settings/searchEngines) by rightclicking the https://duckduckgo.com search field and selecting create a search engine. I got a big list of other custom search engines and neither one has an option of "make default".

It's supposedly possible in Chrome:
https://support.google.com/chrome/answer/95426?hl=en

Is there anything I'm missing?