Opera limited the number of defaut engines to five because of concern that malware was hikacking users to unsafe sites.
http://forums.opera.com/topic/2473/default-search-engine/6
This has been discussed in multiple threads. You might want to research the forum instead of posting a duplicate thread. For the moment, you can type a d in front of your search from the main address bar.
Or, go the extension route, there duckduckgo extensions for Blink. They were approved by Opera as providing needed functionality for the blink browser (which Opera 22 is) ,so just click on Get Extensions from the main menu of Opera 22 and try a DDG extension. For example, here's a DDG extension. https://addons.opera.com/en/extensions/details/duckduckgo-for-opera-2/?display=en
I see the last update was July 25, 2013, so it was done in the last year.
Finally, you can select the Disconnect Search extension (from the Opera add-on site). DuckDuckGo is one of their five default search engines, and with Disconnect Search, if you select DuckDuckgo, you can search with it from the main Opera Address Bar (making unnecessary your typing the D. Also Disconnect searches even with Google, and Bing are safer, because the search query is no longer traceable to you -- or so reviews of this extension (and its developers) seem to say. Only problem with that approach for ddg right now is the Image search with DuckDuckGo isn't working. They say they are looking into that and will fix it. Otherwise, the DDG search, without the image search, works. The image search is working for them with their other search engines.