Exterior plywood contracts and expands, which makes it hard to bond with other materials. In order to install tile directly over the exterior plywood, you will need to use a special bonding agent in order for it to hold up. You will need to purchase a polymer mortar that meets required standards. An alternative to polymer bonding is Wonderboard or a similar product, which allows you to apply it directly to the plywood with adhesive, and then tile on the other side. Either method will give you a stable surface for the expansion and contraction during climate changes.