Combine the rolled oats, mashed bananas, cinnamon, and sweetener (if using) in a mixing bowl, stirring until well mixed.
Pour the milk into the bowl with the dry ingredients, and stir thoroughly until everything is evenly coated and starts to thicken slightly.
Transfer the mixture into a jar or container with a lid, spreading it out evenly and closing tightly.
Place the jar in the refrigerator overnight or for at least 8 hours, allowing the oats to soak and absorb the flavors completely.
In the morning, open the jar and give the oats a good stir to combine everything—notice how creamy and thick it has become, with the oats swollen and tender.
Scoop out portions into bowls, top with your favorite fresh fruits, nuts, or a drizzle of honey if desired, and enjoy the cozy aroma and wholesome texture.