Who should be the owner of oraInventory in such a scenario ? grid or oracle user
Better choose grid if you use role separation.
Where should oraInventory be placed ? Under /grid or /u01 mountpoint ?
As long as both oracle and grid users have rwx privilege on the inventory directory, you can place it on a OFA-Compliant or near OFA-Compliant location.
I personally prefer /u01/app/oraInventory
The permission should look like below
$ ls -l /u01/app/oraInventory
drwxrwx--- 6 grid oinstall 4096 Aug 13 2012 /u01/app/oraInventory
Its better to se a central inventory for both GRID_HOME and ORACLE_HOME. Its easy for opatch auto to run smoothly with a properly configured central inventory.
Both grid and oracle users must have the Oracle Inventory group oinstall as their primary group,so that each Oracle software installation owner (grid, oracle) can write to the central inventory.