As of 04/2024 it looks like under Install OS > Custom > Debian 12 when you click change partitions you can select ZFS for boot and data
Ubuntu does not list ZFS in OVH WebGUI but you bring your own Linux This way OVH can install some of its management tools too (also configures IPv6)
OV BYOL requires a QCOW image (Quemu style container) Ubuntu calls these "Cloud images" and you download it from here to your local machine The default cloud image does not contain a file required by OVH that is for custom things only