Playing with creating extensions for Opera 15, I tried to make one that would re-create the Opera 12 Notes UI and functionality.
As this is not yet an officially submitted extension, you'll have to unzip it, enable developer mode in the Extensions overview screen and click the "Load unpacked extension" button to locate the folder and use it (thanks Rafael Luik). Paste your notes.adr contents in options page to import your data.
There are bugs. Feel free to report them here:
..or comment. Not sure how much time I'll be able to spend on it, but I'll try to solve some issues.