VMware Server 1.x is alive and well and still works fine with a CentOS/OEL/RHEL 5.x host and CentOS/OEL/RHEL 5.x guests
If you’ve ever struggled with VMware Server 2.x security configuration you should find VMware Server 1.x security model much easier !
According to TommyReynolds on the Oracle Forum the Oracle Xen VM (OVM) is a virtualization platform and is a separate product (to OEL & RHEL) with different rules.
With Oracle Enterprise Linux JeOS (Just Enough OS) — a secure, minimized OS that is freely re-distributable and backed by enterprise-class support — developers and ISVs can now easily build Oracle VM Templates.
Oracle OEL JeOS AMI VMs for Amazon EC2