The website for OpenGroupware is of course:

http://www.opengroupware.org/

You will see some screenshots there of OpenGroupware working with Apple iCal. Since the project relies on Cyrus there is nothing preventing its use on Mac OS X Server 10.3.

Regarding Mac OS X installation, their FAQ as this to say:

[quote:a2a228570c]Most of the core libraries already work on MacOS X. Hopefully someone will step in and complete a MacOS X port (if you are interested, write an email to [email protected].[/quote:a2a228570c]