Preheat your oven to 350°F (175°C).
In a mixing bowl, combine the rolled oats, baking powder, cinnamon, and salt.
Add the mashed banana, milk, and vanilla extract to the dry ingredients.
Stir until well mixed and let it sit for a few minutes.
Pour the mixture into a greased baking dish.
Top with mixed berries and nuts if using.
Bake in the preheated oven for 25-30 minutes until golden brown.
Remove from the oven and let cool for a few minutes.
Slice and serve warm, drizzled with maple syrup if desired.
Enjoy your delicious baked oats!