I have always had trouble with dealing with multiple oraInventories and multiple oraInst.loc files in a server.
If there are lots of people installing products in a server or if you are handed an environment which is in such a mess it is a big trouble to get things tidied up (if the installer people used a central inventory instead of each one using their own inventories, you a re lucky).
You need to know which oraInst.loc points to which oraInventory and which products and versions are in that inventory. You need to put the right oraInst.loc in place in order to install a patchset or a patch. Or you need a way to make the central inventory know about all Oracle homes which exist but which are not in the inventory.
In such situations Oracle Universal Installer 10.2 comes to the rescue. There is a Metalink note I have just realized which is 438133.1. The note explains how to consolidate multiple Oracle homes in one central inventory. There is a new option of the installer in 10.2 which is -attachHome. Using a 10.2 installer and this option you can get all your homes included in the central inventory.
./runInstaller -silent -attachHome ORACLE_HOME=<home you want to add> ORACLE_HOME_NAME=<unique home name>
The steps are clearly explained in the note which basically are; using a 10.2 universal installer, setting oraInst.loc to point to the inventory which you want to use to consolidate everything in, setting $ORACLE_HOME to the home you want to include and running the installer with the -attachHome option.
This works for 9i and 10G.