OOIS UML is thoroughly described in the book:

D. Milicev, “Model-Driven Development with Executable UML,”
Wiley (Wrox), July 2009, 816 pages, ISBN 978-0-470-48163-9
Contact: To ask questions or give comments, write to info@ooisuml.org.
History
OOIS UML is invented by Dr. Dragan Milicev, professor at the University of Belgrade, Faculty of Electrical Engineering. The early ideas emerged in late 1990s, while the approach was shaped in early 2000s. Since 2000, the approach has been applied in about a dozen commercial and industrial applications.
Known Implementations