In desktop Opera
(1) Add DuckDuckGo to your desktop Opera by following these instructions
(2) Click the "Opera" button and select "Synchronize Opera". Click "Enable Synchronization". Follow the instructions.
In Opera Mini
(1) Click the, "O" icon in the bottom-right of the app and select, "Settings."
(2) Turn Opera Link on and enter your Opera Sync credentials.
(3) After your login has completed, select the search box in the the top right.
(4) Change the search engine to DuckDuckGo.
Your Opera Mini will now have DuckDuckGo set as its search engine! If you turn off Opera Link in the app settings, DuckDuckGo should still remain as an optional search engine that you can select in the search bar (Step 3).