Using cooking spray or butter, grease the sides of the slow cooker so the oatmeal won't stick. Cut 4 tbsp of butter into small pieces and lay them out on the bottom of the slow cooker.
In a medium bowl, combine diced apples, brown sugar, cinnamon, salt and nutmeg, and stir to combine. Spoon apples on top of butter. Sprinkle oats on top of the apples and then pour the water over the oats.
Don't stir the mix. Turn the slow cooker on low and cook for 9 - 11 hours.