Redhat has an excellent entry on Open vSwitch: Overview of 802.1ad (QinQ) Support.
A key point raised is:
Enabling QinQ support in OVS is a simple configuration change. You must set the vlan-limit option to a value of 0 (unlimited) or 2. By default vlan-limit has a value of 1, which means only one VLAN tag will be inspected. This preserves backward compatibility with older OVS releases.$ ovs-vsctl set Open_vSwitch . other_config:vlan-limit=2
The article continues with ovs-ofctl and ovs-vsctl examples.