Just noticed this old post, but in case it helps, yes you can run a nested Oracle VM Server as a guest VM within vSphere. I set it up a year or so ago in ESXi 4.1 and OVM 2.2.1 - the guest had 2 vCPU and 6GB vRAM. I just booted the new guest VM using the OVM installer and don't remember having any problems installing it. I had OVMM running in a different ESXi guest VM.
Of course this was just a lab environment for experimentation purposes (like people are doing nesting vSphere too) and you wouldn't want to run like this in production.