If you want to add new dba user as domain user, you have to add ora_dba group on Windows Systems. If you do not add to this group, you get an error like ORA-01031 insufficient privileges.
Error
Solution
Domain admins or local admins can add new user to ora_dba group.
If you use linux system, you can add user to dba group with usermod command.
[root@test /] usermod -a -G dba username