Hi mrdl,
OSClass is written in PHP for the server side, HTMl+JS for the client side and MySQL for the database connection.
It's better to use PHP 5.3, but since it's not as widely extended as we want, we try to make it compatible with 5.2.
MySQL is the latest version, but I don't think we use anything that's not included in previous.
About JS we base most of the code on JQuery library, a well known library.