Wow, you're the first person I hear wishing to set RS as default! Except for myself of course :)

I haven't tried Opera, but in Chrome you can't either. The only thing you can do is add RS as 'another' search engine with a shortcut. So I set 'r' as shortcut, and now I type 'r' 'tab' and Chrome switches to RS in the address bar. Maybe Opera has some similar settings?

Please let me know if there is anything you'd like to have improved in RS!