Hello,
The next version of opera is based on chromium, and have (almost?) the same API. Tried the .crx version with Opera Next and it works fine (except some warnings) so there no much work to be done before it become ready.
I think this will reduce development efforts in the future.
Opera 15
Opera 12 is most recent standard release, however... Opera Dev's announcement...
From Opera 15 onward, Opera 11 & 12’s extension format is no longer supported, and instead, we’ve switched to Chromium’s extension model. Check out our new documentation for developing extensions for Opera 15 and higher and start building your own extensions.
http://dev.opera.com/extension-docs