Your best bet is to use the VMware Standalone Converter (latest version available) and select "Powered on machine" as the source. The converter can convert RHEV based Linux VMs to VMware ESXi5 VMs which can then be exported to OVF. I've used this method and it works just fine. You simply have to do the multiple steps since there is no tool I know if that will take you directly from RHEV VMs to OVF format directly.
I'm using VMware Standalone Converter v5.5 and it's working well.
Hope this helps.