Opera Mini works by connecting to proxy servers that filter Web content so that it displays properly on a cell phone's tiny screen. All Opera Software would have to do is run a separate set of proxies for "Opera Mini China" and then actively refuse connections on their main proxies originating from Chinese IP address blocks. (Or, more likely, the Great Firewall will block them.)
The China version of Opera Mini was already using a transcoding server located in China. But people could install the international version if they wanted. What has changed now, is that usage from inside China of the international transcoding server is not supported anymore. It would indeed be trivially easy for the GFW to block this server in Oslo. But that didn't happen, instead the Chinese users now get a page asking them to use the the China Opera Mini version.