First, you need to remove the chrome as a custom search engine by:-
Unchecking"make this default" by going to the
-chrome settings > search engines in the browser.
Chrome displays two groups of search engines: the default search settings, all engines that Chrome ships with, or were added by you as default search engines, and the other search engines.
You also get to choose the default option when you first download a browser.
Now, change the default search engine in opera by-
opening opera:> settings> search engines in the browser's address bar to do so. Check and make it the default browser.