Can I have same ORACLE_BASE location for Grid (CRS) and RDBMS software Homes ?
Yes, You may have.
You can have the same Oracle base, while installation you can install GRID and RDBMS on different directories such as:
assuming /u01/app/oracle is your Oracle Base
- /u01/app/oracle/grid -- for grid
- /u01/app/oracle/rdbms -- for database
The OFA guideline for ORACLE_BASE is /u01/app/<owner>. If you are using the same owner for all software (let's call him "oracle") then a simple installation will have one base /u01/app/oracle. This base will be shared by all installations of Grid and RDBMS software. But, if you use two separate owners (let's call them grid and oracle) then you will have two separate ORACLE_BASE locations /u01/app/oracle and /u01/app/grid. I always use two separate owners, but I know many people who only use the oracle account and that is fine too.
Oracle allows every ORACLE_HOME to have a unique base. There is no requirement to install more than one ORACLE_HOME under a given ORACLE_BASE location. This is just an extension of what I said above, that the ORACLE_HOMEs for Grid and RDBMS can be stored under separate ORACLE_BASEs.
If you are using RAC then I strongly suggest you read chapter 1 of the Oracle Real Application Clusters Installation Guide. Read section "Overview of Oracle Base and Oracle Home Directories". The rules for ORACLE_BASE are as I noted above, but the rules for ORACLE_HOME change: the Grid ORACLE_HOME is not to be a sub-directory of the Grid ORACLE_BASE as it is when not using RAC. So, with RAC your directory structure might look more like this:
mkdir -p /u01/app/oraInventory
# Grid Base
mkdir -p /u01/app/grid
# Grid Home
mkdir -p /u01/app/22.214.171.124/grid
# Oracle Base
mkdir -p /u01/app/oracle
# Oracle Home
mkdir -p /u01/app/oracle/product/126.96.36.199/db_1
# Ownership for all of the above...
chown grid:oinstall /u01/app/grid
chown grid:oinstall /u01/app/oraInventory
chown grid:oinstall /u01/app/188.8.131.52
chown grid:oinstall /u01/app/184.108.40.206/db_1
chown oracle:oinstall /u01/app/oracle
My examples show 220.127.116.11 but it's the same for 18.104.22.168.
There's no requirement to create the home directories prior to running the OUI. It can create them for you. I just shown these commands to help you understand the big picture.
The OUI will pop-up a warning if the Grid user's ORACLE_HOME is outside of his ORACLE_BASE as shown in the above mkdir commands, which is funny since it is the recommended setup in the Oracle RAC documentation. I submitted a bug report several years ago. To be consistent across all of my RAC and non-RAC servers I follow the more strict rules of RAC and use the mkdir commands shown above.