Combine: In a 12-16 oz. wide-mouth mason jar, add the oats, yogurt, milk, mashed banana, chia seeds, vanilla, and cinnamon. Add nuts and sweetener if using.
Mix: Stir the ingredients thoroughly until well combined and the chia seeds are evenly distributed.
Set: Secure the lid and refrigerate for at least 4 hours, though overnight is best for the creamiest texture.
Serve: Remove from the fridge, give it a quick stir, and garnish with fresh banana slices, extra nuts, or a dash of cinnamon.