For that, I created Oracle user and installed oracle on oracle user. root or any other users do not have permission to access oracle. I want to set permission for the root as well as for other users to access oracle.
I recently installed Oracle 10g Express Edition (the free version of Oracle) on an Ubuntu machine. To be able to start and stop the database and do other things, I had to make myself a member of the 'dba' group. Read the Oracle documentation.
That's right. Since Oracle itself is an OS process, if you want to control the Oracle server, you need appropriate OS privileges. If you just want to work inside of Oracle, you need an Oracle user acount. You can have either one or both, depending on how you need to interact with Oracle.