Add the banana chunks to the bottom of a 16-ounce wide-mouth mason jar. Using a wooden spoon, press down on the chunks to smash.
Add the dry ingredients to the jar.
Top with kefir and milk of choice. Stir to combine.
Store in the refrigerator overnight. If too thick for your liking, add a splash of milk and stir to combine until you reach your desired consistency.
Top with chopped peanuts and sliced bananas, if desired.
Notes
I use a vanilla protein powder that is sweetened with stevia leaf extract, which adds a subtle sweetness to the overnight oats. If you're using an unsweetened protein powder, maple syrup is a great option to sweeten the oats.
I used plain, unsweetened kefir with 1% fat in this recipe. Fruit-flavored kefir is typically high in sugar, and I like to be able to control the amount of sugar that is added to my oats.