How to make oat flour:
Real way: obtain whole oat grains, ideally by going into an organic field with a sickle, while singing Kumbaya. Grind them in a mill.
Lazy/easy way: Put oatmeal (dry, uncooked, plain, old-fashioned or quick) in your blender. Turn it on. Watch it turn into powder, which you now name “oat flour.”
Seems too simple to be right, but for most purposes, it will work just fine.