OS X does not run on X11, but on
Quartz, the proprietary engine for the graphics system. However, X11 is provided in the installation discs (if you're using at least 10.2 or greater) but is not installed by default. When installing OS X, you have to select "Customize" and then you'll be presented with the X11 selection along with some other options. You can also install it by inserting the install disc and looking for the X11 package. It can also be
downloaded from Apple.