When OpenOffice 3.0 is released sometime next year (target date is September) it will be able to run natively on Mac OS X. Right now OpenOffice can run on the Mac OS but before being able to do this you need to install X11, a Unix windowing software. With the Mac OS X version of OpenOffice this is no longer necessary.
For those who can't wait next year there is already a native Aqua version of OpenOffice, called NeoOffice, which works just fine and can be downloaded for free. The NeoOffice developers are not affiliated with OpenOffice.org.