Hoof oil can go anywhere on the horses hoof but I have been advised by multiple trimmers that putting oil on a horse's feet is utterly pointless, you cannot improve a horse's horn growth from the outside, only the inside. If you're in a dry climate you can however soak the feet in water to allow them to moisturise themselves. The hooves are amazing for doing what they need to. :)
Yes it can go anywhere on the horses hooves and anywhere that can get dry and brittle. Like the sole, frog and cornet band.