In a pot, add the 4 cups of water and cinnamon stick. Bring to a simmer at medium heat. Once it comes to a simmer, let it cook for 10 minutes.
Pour in the oats. Stir well to combine. After it comes back up to a simmer, pour in the milk and sugar, to taste. Bring it back to a simmer and cook for 10-12 minutes or until it becomes thick.
Serve with your favorite sliced fruit or plain. Pour in a little more milk and stir well to combine. Occasionally, I enjoy garnishing with crushed piloncillo sugar cane.