OVN is Red Hat’s CNI for pod networking. It is a Geneve based overlay that requires L3 reachability between the host nodes. This L3 reachability can be over L2 or a pre-existing overlay network. Openshift’s OVN forwarding is based on flow rules and implemented with nftables on the host OS CNI pod.
For more information, see About the OVN-Kubernetes network plugin.