ovf doesn't seem to be supported but vdi is, which is the Virtualbox disk image format. So if your goal is to import the openstack virtual machine into Virtualbox you could also probably do the following (Doesn't Virtualbox support qcow2 disk images out of the box?):
I've made the following assumptions: You're not using cinder as storage for the virtual machine you are trying to import into VirtualBox.
First we need to get the instance name of the virtual machine you want to import:
nova list
nova show <uuid of instance you want to export>
# Note down the name of the instance which should be something like instance-00000xx
Now we need to go the actual place where the instance is being stored
cd /var/lib/nova/instances/instance-00000xx # change this to actual instance name
qemu-img convert -O vdi disk virtual-box-disk.vdi
Now you can go to Create a new virtual machine and choose use existing disk: virtual-box-disk.vdi.
Haven't verified yet if this will work. Also the paths may be different if you are in devstack or on a different OS.