In a large non-stick pan over medium-high heat, melt the butter. Add the diced apples and cinnamon and stir to cook them down for 3 to 5 minutes until they've softened. Once they're soft, turn off the heat, and pour the maple syrup over the apples. Stir to mix.
Assemble:
Fill each jar with ½ cup oats and ¾ cup milk, then stir. Divide the cooked apple mixture evenly among the jars. Stir to combine the cinnamon apple mixture with the oats, then cover the jars with a lid.
Refrigerate & serve
Refrigerate the jars for 8 hours or overnight. These keep in the fridge for up to 4 days.
Enjoy the overnight oats cold from the fridge or warmed up for 30 seconds in the microwave.