ok, I'm not sure on any mpkg.
The file you want is: ooo103darwingm.dmg
from here:
http://porting.openoffice.org/mac/ooo-osx_downloads.html
this is a dmg file which when double clicked on shows the following (at least for me):
An alias called Install_OpenOffice.org, a folder named InstallerData, a folder named MacOSX, a read me README_OOO.html, and another folder called Third-Party Source
Double click on the alias to Install_OpenOffice.org and it opens up the app InstallAnywhere (also used by limewire, if your ever played around with that).
The third party folder has a little app you can install called Start_OpenOffice, which will let you start open office via an icon, instead of by command line.
As far as running linux under X11 well I think you may have the wrong idea about what X11 is, X11 is a graphical user environment for *nix based machines.
http://www.apple.com/macosx/features/x11/
X11 will allow you to run applications that were originally built for *nix machines, either by using a great little program called fink, or by compiling from source.
Compiling from source can be a great learning experience, but can also be frustrating, if you don't know what flags need to be set/unset to compile an X11 app under OS X (This isn't unique to just OS X, as even different flavors of *nix OS's need different flags for compiling).