One size does not fit all.
The Oracle VM template comes with just the twm windows manager. Since my days in University I dislike this thing. For me it is Motif.
So in order to get my favorite Windows manager up and running I need to install it.
Luckily Oracle offers a public yum server. Do the following:
Edit this file and set the stanza to enabled for the corresponding entry. The template is based on EL5 Update4.
Then just run
yum install openmotif
Afterwards I modify the xstart file in the /home/oracle/.vnc directory to use mwm instead of twm.
In order to access the vncviewer I also need to modify the iptables.
Add a line into the iptables to enable access in the port range 5900-5950. This will enable the access of the custom vncserver, as well as the vncserver which runs on the VM Server. The second vncserver acts as a console for the VM image. Depending on the order in which the VM's are started the vnc port changes. Therefore I use a port.