Opera has three built-in options for search:
The address bar, where entering any query that has a space automatically searches using the default search engine. If you want to enter something that doesn’t have a space, prepend it with either "? " or the letter of the search engine you want to use, e.g. "w " for Wikipedia, etc.
The second is, of course, the search bar, usually defaulted to the right of the address bar. In Opera, not only can you select different engines using the dropdown box, but you can also drag the left separator to the left to lengthen it, or to the right to lengthen the address bar.
The third is customizable search for the address bar. I recently set up a custom search for NeoGAF so I don’t have to constantly enter "g site:neogaf.com " in the address bar, merely "n ".
How to set it up:
You can do this for any site or any search engine, in fact.
Edit: There’s actually a fourth option, the Speed Dial page has a search engine. I set this to be Wikipedia, since I use it often, but not often enough to make my default. My default, Google, is the default in the address bar and in the search bar.
As seen below: