You can install OpenVPN on MacOS X very easily using MacPorts. Once you have it installed, any guide to configuring OpenVPN will show you the next steps. The official OpenVPN howto is a good place to start.
One of the key things you'll need to do is enable your MacOS X server to route packets between interfaces. This can be accomplished with the following sysctl command:
sysctl -w net.inet.ip.forwarding=1
This does not survive reboots, so you will have to make OpenVPN invoke it, or make a
launchd script to do it.
Remember also that, once you become a router, you should be filtering packets. MacOS X has offered
ipfw packet filter for a long time, and more recent versions include the even nicer
pf packet filter. Documentation of both of these tools is abundant, including in MacOS X