Preheat your oven to 350°F (175°C).
In a large bowl, combine rolled oats, brown sugar, baking powder, cinnamon, and salt.
In another bowl, whisk together milk, melted butter, eggs, and vanilla extract.
Pour the wet ingredients into the dry ingredients and mix until well combined.
Gently fold in the mixed berries.
Grease a baking dish with butter or cooking spray.
Pour the oatmeal mixture into the prepared baking dish.
Bake in the preheated oven for 30-35 minutes, or until golden brown.
Remove from oven and let it cool slightly before serving.
Serve warm, topped with additional fruit or honey if desired.