My mom gave me some Scottish soap with coarsely ground oatmeal in it. It hurts so it's only hand soap lol. Keep that into consideration. I prefer it finely ground personally.
Fine. Very fine !! Scratchy is ok for garden soap Like the idea of oatmeal in soap but don't intend to do a second batch, however it does have label appeal.
When making most of my oatmeal soaps I also use baby oatmeal then put it in my bullet with whatever milk I happen to be using for the batch. On the other side of the spectrum I make an extra scrubby mud bar with lots and lots of whole quick cooking oatmeal that I regularly sell out of and customers come back for more. I do find the quick whole oats are not as sharp as grinding oatmeal. I do not like steel cut oats even in my extra scrubby soap
My preference is colloidal oatmeal (which is NOT another name for finely ground oatmeal). Colloidal oatmeal is a specially prepared form of oatmeal that mixes better with creams, lotions,http://dermnetnz.org/treatments/topical-formulations.html and water. It is produced by finely grinding the oat and boiling it to extract the colloidal material.
When I can't get real colloidal oatmeal, I like Gerber babyfood oatmeal - it is so finely ground it is like a powder.