In Star Office there is a start-up script that brings you to a menu window that gives the option to uninstall. I think that Open Office has the same feature. If I get time later, I will boot into Mandrake and try to find it for you.
P.S. OK since you installed from source, it may be different for your system. But look in the source directory of Open Office, where you unpacked the app with the file manager.
Look for an icon called setup. Double click on this and it should bring up a menu that allows you to uninstall Open Office.
It could be under /usr/lib/openoffice/ as well, or in your home directory.
If you canīt find it, in a console, type openoffice. Look for the path to the library or main program, then look for the setup icon.