Oatmeal is a natural humectant that helps skin retain its natural moisture, and  helps reduce irritation from inflamed, dry, itchy skin. It softens, keeps skin soft, and is even safe for babies. 
According to Wikipedia:  The earliest recorded evidence of the production of soap-like materials dates back to around 2800 BC in Ancient Babylon.[5] In the reign of Nabonidus (556–539 BCE) a recipe for soap consisted of uhulu [ashes], cypress [oil] and sesame [seed oil] "for washing the stones for the servant girls".[6] A formula for soap consisting of water, alkali, and cassia oil was written on a Babylonian clay tablet around 2200 BC.