Oats in their whole form are called groats. Oak groats are effectively undigestible by humans. But when "rolled" or "cut", they are partly broken down physically so your body can use them.
Oatmeal is just that oats that have been rolled or cut. You can store oat groats, but to use them you need a "roller mill".. it's a uni-tasker..
After rolling you separate the hulls from the grain.. eat.