Hi,
In general, it's not a good idea.
And practically speaking, OCS is a 32-bit app and should run on Windows 2003 server 32-bit. Exchange 2007 is a 64-bit app and has to run on the 64-bit version of Win2k3. Because OCS and Exchange 2007 require different operating systems, I would advise against installing them on the same box, even in a demo scenario. For demos, I would just use 2 different virtual machines. For production, I would have 2 separate boxes.
Regards,
Matt