Porridge is a hot dish made by boiling grains or legumes in milk or water until thick and soft, often eaten at breakfast. Oatmeal is a well-known type of porridge made specifically from oats—rolled, steel-cut, or instant. Other porridges include corn-based grits and polenta and rice-based congee. In short, all oatmeal is porridge, but not all porridge is oatmeal.
Source: Encyclopaedia Britannica — porridge.