Place the ripe banana in a medium mixing bowl and mash with a fork until mostly smooth.
Add the milk, Greek yogurt, maple syrup, vanilla extract, cinnamon, and salt. Stir until mixed.
Add the rolled oats and 2 tablespoons of chopped walnuts. Stir until combined.
Divide the mixture evenly between 2 mason jars or airtight containers.
Cover and refrigerate for at least 4 hours, or overnight, until the oats soften and thicken.
Stir the oats before serving.
Top each serving with sliced banana and the remaining chopped walnuts.
Notes
Tips and Tricks
Choose Ripe Bananas – Bananas with brown spots are sweeter and easier to mash, making them perfect for overnight oats.Adjust the Thickness – If your oats are thicker than you like, stir in a splash of milk before eating.Keep the Walnuts Crunchy – Store the walnuts reserved for topping separately until serving time if you prefer a firmer crunch.Add Banana Just Before Serving – Fresh banana slices taste better and won't turn brown while the oats are refrigerated.Use Containers With Lids – Mason jars or airtight containers make these oats easy to store and take with you.
Storing
Refrigerator – Store the overnight oats covered in the refrigerator for up to 3 days. Keep the fresh banana topping separate until ready to serve.Freezing – Freeze the prepared oat mixture without the fresh banana slices or reserved walnuts in freezer-safe containers for up to 2 months. Thaw overnight in the refrigerator before serving. The texture may be slightly softer after thawing.