- Add a new rule to your firewall (System Prefs -> Sharing -> Firewall -> New) with the port name = "X11" and the port number = "6000"
- Start X11
- In your xterm type xhost +
$ DISPLAY=myosxmachine.foo.com:0.0to run an instance of the image viewer xv on the OSX machine's X11 server.
$ export DISPLAY
$ xv
What you've done is to A) Open the firewall for port 6000 (the X11 port) and B) Allow anyone to use your X server (xhost+). Be aware of the implications of doing this however, and make sure you read up on the 'xhost' command and how to limit access only to the hosts you trust (tyep man xhost).
[Editor's strong caution: Please see the comments for why this is a Bad Thing to do! The right way to use this command is with an actual IP number or host address in the xhost command, as shown in the comments. You can also use the ssh syntax shown in another comment ... my apologies for running this as is without testing and researching first.]

