Lightly coat the inside of a 4-5 quart slow cooker with cooking spray.
Combine all the ingredients in the slow cooker and stir.
Cook for 7-8 hours on low. Stir thoroughly before serving with a rubber spatula or wooden spoon. Serve immediately.
The leftover oatmeal can be portioned into tupperware-like containers and refrigerated to be reheated and enjoyed for 3-4 days.
Notes
Oats: steel cut oats are essential in this recipe. Quick oats or regular rolled oats won't hold up to the long cooking time in the slow cooker. Liquid: also, the liquid measurement completely depends on how you like your oatmeal. My sister, who gave me the recipe, cooks her oatmeal in the slow cooker with 8 cups water to 2 cups steel cut oats but she doesn't add any milk or liquid in the morning when serving. I, on the other hand, like a sturdier oatmeal consistency because we pour a bit of milk on ours when serving, so I use 6 cups water to 2 cups steel cut oats. Judge the liquid-to-oats ratio based on how you like to eat your oatmeal.