There's also Dr. Barry Alberstein. He developed the "CAM" system - the electronic / online version of OCS. Alberstein Consulting - Achieving Independence by Focusing on the ControllableBesides the book from NU mentioned above, here is another great resourse I found that explains the system.
One Card System Expert | Jim Marker
Under "products", there are a number of free recordings that explain the system. It looks like the author is an NML GA.