http://www.computerworld.com.au/article ... -natively/
When it comes to deciding which ads to block, Opera isn't reinventing the wheel: It's using the same blacklist as the browser extension Adblock Plus, EasyList.
Unlike Adblock Plus, the Opera browser doesn't use a whitelist of acceptable ads, because it starts off with ads enabled for each site. It's only once the user has decided that ads on a given site are unacceptable and clicked on a button in the toolbar that they are blocked.