Combine the oats and water or nut milk in a bowl. Mix in the maple syrup, apple cider vinegar, and cinnamon. Let it sit in the refrigerator for 4 hours to overnight.
Take the oatmeal out of the refrigerator. If using steel cut oats, put them in the blender and blend until it reaches an oatmeal-like texture and consistency. Transfer it into a bowl. You do not need to blend the mix, if you are using raw rolled oats.
Dice the apple and stir them into the oatmeal.
Serving cold: Top with sliced apples and cinnamon and serve.
Serving warm: Put it in a sauce pan or a pot and heat on low, stirring constantly. If it gets too thick during this, add water or more nut milk to thin. Check the temperature with a candy thermometer and turn off the heat when it reaches 100°. Immediately transfer into a bowl. Top with sliced apples, some cinnamon and serve.