The correct answer is "Undo the changes you made during the install"
By this I mean:
Stop all of the processes
Delete the ORACLE_BASE directory (usually the one one level up from ORACLE_HOME)
Delete the oraInventory directory if it isn't inside ORACLE_BASE
Delete /etc/oratab, /etc/oraInst.loc
Delete /user/local/bin/coraenv, /user/local/bin/oraenv, /user/local/bin/dbhome
Undo any environment settings you may have modified (in .bash_profile and bashrc)
That should be just about it. That's how I usually clean out my system between installs.
You don't need to run OUI, you can do this manually also. Here are the steps which I follow:
Stop all the databases and listener on the box either manually or using your startup script (/etc/init.d/oracle stop).
rm -rf /etc/ora*
rm -rf $ORACLE_HOME
rm -rf $ORACLE_BASE
rm -rf /tmp/.oracle
rm -rf /var/tmp/.oracle
This should be enough to remove the whole software from the box, you can drop the OS users and their home directories also if you want.