The avenanthramides in oatmeal work as a skin protectant and protective barrier by restoring the top layer of skin (Source). By helping to heal the outer layer of skin, not only can more moisture can be retained, but also it will keep infection and irritants out.
When ground, oatmeal can gently buff the top layer of skin.
Soothes Irritated and Inflamed Skin
Avenanthramides are the principle polyphenolic antioxidants in oats, and they have been shown to alleviate inflammation and also reduce itching.
Oatmeal absorbs oil without over-drying. Oatmeal has astringent properties and helps gently draw oil out of our pores.
While we don't really have to worry about the cleaning ability of oatmeal when used in soap, nevertheless oats contain natural saponins. Saponins are the chemical structure of soap.