Recently, Opera software released Opera 18 Developer, which today received another update. This build is based on Chromium 1622.7. In addition to fixing crashes and other bugs, Opera 18.0.1264 Developer update has added two new features. One, the Opera menu now opens with mouse down and two, an option to create a right-click search engine has been added to the search field.
Read: Remove unnecessary search engines from Firefox
How to Add DuckDuckGo Search Engine to Opera 18 Developer
Opera already comes with search engines like Google, Yahoo, Bing, Amazon and Wikipedia. For example, for add DuckDuckGo search engine to Opera, Visit DuckDuckGo home page, right-click the search field and select Add as a search engine.
Enter the name and keyword shown for the dialog and click Create button. DuckDuckGo.com will be added and displayed on Other search engines that you cannot make the default.
Download Opera Developer
You can find the complete changelog on here.