Preheat your oven to 350°F (175°C).
In a large bowl, combine the rolled oats, baking powder, cinnamon, and salt.
In a separate bowl, whisk together the milk, mashed bananas, maple syrup, eggs, and vanilla extract.
Pour the wet ingredients into the dry ingredients and mix until well combined.
Fold in the mixed berries gently.
Grease a baking dish with non-stick spray or butter.
Pour the oatmeal mixture into the prepared baking dish and spread evenly.
Bake in the preheated oven for 25-30 minutes, or until golden brown on top.
Let it cool slightly before slicing into squares.
Serve warm and enjoy!