Combine all ingredients in the slow cooker. The cinnamon should be wet but will float on top.
3 cups old-fashioned oats, 2 cups milk, 4 cups water, ⅓ cup pure maple syrup, 2 tablespoons cinnamon, ½ teaspoon salt
Cook on low for about 4 hours. About 2 hours into cooking, lift the lid and stir to mix the cinnamon in, then replace the lid and continue cooking for the remainder of the time.
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Notes
Old-fashioned oats work best for slow cooker oatmeal because they hold their shape and texture well during the long cooking process. Quick-cooking oats will become too mushy.
To prevent the oatmeal from sticking to the sides and bottom of the slow cooker, lightly grease it with cooking spray or a small amount of butter.
You can use other sweeteners like honey, brown sugar, or agave syrup instead of the maple syrup. Adjust the sweetness to your liking.
You can add other spices like nutmeg or a pinch of cloves for extra flavor.
Resist the urge to stir the oatmeal more often than once, as additional stirring can cause the oatmeal to break down too much and get mushy.
About 30 minutes before the oatmeal is done, you can add various toppings and add-ins like chopped nuts, dried fruits, fresh berries, or sliced bananas. This adds texture and extra flavor.
For creamier oatmeal, use more milk and less water.
If the oatmeal become too thick as it cools, simply add additional milk.
If you want to have oatmeal ready in the morning without using the slow cooker, you can prepare overnight oats by combining all the ingredients in a jar or container and leaving it in the refrigerator overnight.