NEVER add oatmeal to your meatloaf, if you want to stretch the recipe add dry bread crumbs.  I detest meatloaf with oatmeal as a filler.  Ewww.  There are other things you can put in to make it go farther too but oatmeal is just gummy and disgusting, even UNCOOKED.  No one I know of uses already cooked oatmeal, but still - it's gummy after it bakes and it's gross.  It's merely a means of trying to stretch the meat.  Not acceptable.  A bad idea.