I wrote a little about the architecture of OVN and how to set it up on multiple nodes here:
That allows you to use lxc network create mynet --type=ovn network=<uplink network>
However the network you specify as the uplink network is will based on your own requirements, i.e do you want the OVN network’s router connected to an existing LXD bridge network, or to an external physical network.
For the physical network uplink (Linux Containers - LXD - Has been moved to Canonical) you need to set ipv{n}.ovn.ranges to allow the OVN router to allocate an address on the uplink network, and dns.nameservers for the OVN router to include in DHCP responses to OVN NICs.