OSDBA group
Members of this group have database administrative privileges (that is: the SYSDBA privilege)
The default name for the OSDBA group is dba.
OSOPER group
Members of this group have some limited database administrative privileges, such as starting up and shutting down the database (That is: the SYSOPER privilege).
The default name for the OSOPER group is oper.
Creating the oracle user
useradd -g oinstall -G dba,asmdba,backupdba,dgdba,kmdba oracle