Mash the ripe banana in a medium bowl or directly in a clean jar until mostly smooth.
Add the milk, maple syrup or honey, vanilla extract, and mashed banana. Stir until well combined.
Add the rolled oats, cinnamon, salt, and chia seeds if using. Stir in the Greek yogurt for a creamier texture.
Mix thoroughly until the oats are evenly coated and there are no dry pockets. Add a small splash of milk if the mixture seems too thick.
Cover the container tightly and refrigerate for at least 6 hours, preferably overnight.
In the morning, stir the oats well. If they are too thick, add one or two tablespoons of milk and mix again.
Top with fresh banana slices, berries, chopped nuts, peanut butter, or additional maple syrup.
Serve chilled, or gently warm the oats if you prefer a warm breakfast.