As far as the OFA directories structure is concerned the installation guide recommends:
- to create an "/u01/app/grid" directory to be used as an ORACLE_BASE for this "grid" user;
- to create an "/u01/app/11.2.0/grid" directory to be used as an ORACLE_HOME for this "grid" user.
If you're like me, the above configuration looks a little bit weird because I used to think that the ORACLE_HOME should be somewhere under the ORACLE_BASE directory. Nevertheless, the documentation clearly states the following:
For grid infrastructure for a cluster installations, the Grid home must not be placed under one of the Oracle base directories, or under Oracle home directories of Oracle Database installation owners, or in the home directory of an installation owner. During installation, ownership of the path to the Grid home is changed to root. This change causes permission errors for other installations.
However, the above applies just to cluster installations. If you just want ASM installed for a single instance database then it's fine (and recommended) to place the ORACLE_HOME under the ORACLE_BASE. If not doing so, you'll get the following warning:
So, to sum up the above ideas, remember that if you are going to install a RAC then you need to create the grid ORACLE_HOME out of the ORACLE_BASE of any oracle software owner. If you choose to install the Oracle Grid Infrastructure for a standalone server then the ORACLE_HOME of the grid user should be under its ORACLE_BASE.