Add 1/2 cups of oats to each containter.
Layer in apples.
Sprinkle on cinnamon, adding 1/4-1/2 tsp per jar, depending on how much you like spice.
Pour enough milk to entirely cover oats. If you like a creamier consistency, add up to 3/4 cup per jar.
Cover jars with lids, shake, and refrigerate overnight.
Before eating, heat about 30-40 seconds in the microwave, if desired. If you like sweeter oats, add a bit of honey or maple syrup.