Oracle VM does not take its own snapshots....but in a sense a clone is a snapshot. There is no mechanism to track changes to do any type of rolling snapshots. However your storage may support snapshots. I use them on the storage side.
You can use OCFS2's reflink command for creating snapshots of files (vn.cfg and cirtual disks) on your storage repository, if your repository is located on an OCFS2 volume. I am using this to "archive" VMs files, by creating reflinks from virtual disks to a special folder on my OCFS2 storage prior to deleting them from OVMM.
Now I am using Sun Storage 6180 Array direct attach (Fiber Chanel) with 2 Sun Fire X4100M2 Servers as Cluster, I don't know whether this storage can create snapshot for this or not? How about Oracle VM Manager can help for this?
It seem more difficult than VMware vSphere, if Oracle can create a option for this is very good.