Combine rolled oats, milk, mashed bananas, cinnamon, and vanilla extract in a mixing bowl. Stir well until the mixture is evenly blended and the oats are coated with the liquid and spices.
Transfer the mixture into a jar or a container with a lid, ensuring it's filled with enough space for expansion.
Seal the jar tightly and refrigerate overnight or for at least 4 hours. During this time, the oats will absorb the liquid, and the flavors will meld.
In the morning, open the jar and observe the thickened, pudding-like consistency with soft banana pieces throughout. The oats should appear moist and creamy, with a fragrant cinnamon aroma.
Scoop the oats into a bowl or enjoy directly from the jar for a convenient, ready-to-eat breakfast.
Optionally, top with fresh fruit, nuts, or a drizzle of honey if desired, for added flavor and texture.