Line a 3 1/2- or 4-quart slow cooker with a disposable slow cooker liner. Place apples, cranberries, oats, brown sugar, butter, vanilla extract, cinnamon, and nutmeg inside. Pour in milk and water and mix to combine. Cover.
Cook on Low for 6 to 8 hours. Sprinkle cooked oatmeal with chopped walnuts.
