I would like to use DuckDuckGo as my default search engine in Opera. Opera versions 17+ come with a search engine manager (Menu Settings > Search > Manage search engines...), where you can add custom search engines and change the default search engine (factory setting is Google). Anyway, unlike most other browsers, the default search engine in Opera can be only changed to a different predefined search engine, not to a custom search engine. Without speculating on the reasons behind this arbitrary restriction, I wonder how it's possible to convince Opera to use a non-predefined engine as default. There should be some setting file to edit manually I guess. I've tried grabbing my user data folder ("%APPDATA%\Opera\Opera" on Windows, should be somewhere under ~/Library on OS X) for "google" and "yahoo", but I couln't find the spot.
This functionality doesn't exist yet in Opera Blink (currently on version 19). Remember, Opera went through a major change from Presto (12.x) to Blink (15+), and they're trying to gradually get all that functionality back bit by bit in each new release.
Additionally, there is always the danger that such functionality can be hi-jacked by malware - which is why this functionality was removed in a security fix from Opera 12.14 to 12.15 and wasn't added back in in the latest Presto version, 12.16.
From the Opera 12.15 release notes:
So please don't assume it's an "arbitrary restriction" just because you don't like it. ;-) Also, not too long ago, Opera was the only browser allowing you to even have custom search engines. If you're happy to live with that little security hole due to confidence in your firewall/virus scanner, use 12.14 (you can use that alongside Opera Blink and even right-click on a webpage and "open with" Blink or any other browser); otherwise, you'll have to wait until this gets added back to Opera Blink, or until your favourite search engine gets added to the list of "standard" ones. You can always post to the Desktop wish-list forum.