Sharing this recent experience around enabling X11 on Exadata after running into similarly the same confusion back in 2016 and now in 2018.
First, we had ran into an issue with enabling X11 on Exadata compute nodes in May-June 2016 which was ultimately resolved by installing additional RPMs to support X11.
We had worked with several hardware and software versions of Exadata prior to that and never needed to install the additional RPMs before.
There were other issues preventing DBAs, for example, from running X11 based tools like dbca or runInstaller.
Some of them were due to X11 forwarding not being enabled in the sshd_config file or the firewall rules or the issues with the DISPLAY variable ,
especially when running X11 based tools over the VPN connection.
We had an SR opened for that issue and we were told that there were some recent changes to the Exadata image and several X11 RPMs didn't make it anymore as far as being part of the default Exadata image.
Fast forward 2+ years and we are in a very similar situation in 2018.
More specifically, Exadata machines that were deployed with the image version 220.127.116.11 from January 2018 were "missing" a few X11 RPMs while Exadata machines deployed with the image version 18.104.22.168 from May 2018 didn't require any additional X11 RPMs.
The moral of this story , as I see it, is that this well tested Oracle Support DocID is still your good friend when it comes to troubleshooting X11 on Exadata compute nodes.
Unable to run graphical tools (gui) (runInstaller, dbca and dbua) on Exadata 22.214.171.124.0 - 18.1.X.X.X (Doc ID 1969308.1)