The easy way to change the default search engine in opera is very simple'
first add your favorite searchengine to the searchengines in the list
I have the Dutch version so I'll give the buttons-
in Extra (could be options in english) -
use Shift F12 -
panel choice= buttons -
click; search -
now simple drag and drop the new searchengine to the bar next to the original.
now rightclick on the unwanted original and click remove from the bar
the new searchengine is now the only searchengine and Google is out.