@nebulousecho this has been requested for quite some time now...
either one can add ecosia as search engine or (what i do), i just use this opera addon which allows me to set whichever website i want as default when opening a new tab.
I know it's not ideal, but currently there's not many other options.
Go to About Opera and copy your profile directory (as in, highlight it and Ctrl-C or Copy from the context menu), then close Opera. Open a file browser and Paste your profile's location into the Location bar. You'll see a file there named Local State (no extension) - since it has no extension Windows will ask you what program to use to open it. Something like Notepad or any text editor will work fine. The File menu should include Find, have it search for Location. If you're currently in the US, you can just delete the section (the word Location and the three fields in braces that follow) and Opera will recreate that section when you next open it. Save the changes (make sure "All files" is selected in the Save dialog or Windows will want to add an extension) and you're done.