@agmoyer i have found the solution. this weird problem also happened for me but not with every link. so i thought there must be smth wrong with opera and in the end i was half right.
this bug occurs if you bookmarked that URL. so if you remove that site from your bookmarks URL will be opened as normal behavior not redirects you to google search. just try it.
i believe this is a bug. @leocg , can you try to open a URL after bookmarking it? (bookmark some url, close the tab, then copy and paste that and hit enter) you will see that instead opening that web page opera searching that URL on default search engine.