I recently tried to install oracle under ubuntu. I added a new user and could not start any x-windows as this new user. “xhost +” was the solution.
oracle@orabox:/home/oracle/database$ xclock No protocol specified Error: Can't open display: :0.0 root@orabox:/home/oracle/database# xhost + access control disabled, clients can connect from any host root@orabox:/home/oracle/database# su oracle oracle@orabox:/home/oracle/database$ xclock