All of your clients can start off executing the same script. the program logic then can check to see what OU the client (user and/or computer) resides and execute additiona specific script for that OU. I have an older write up on that implementation on my web site.
http://home.comcast.net/~habullock/logon_script_architecture.htm
Specificially, How-The-OU-Stuff-Works
