Preheat your oven to 375°F (190°C).
In a large mixing bowl, combine the oats, baking powder, cinnamon, and salt.
In another bowl, whisk together the milk, maple syrup, egg, melted butter, and vanilla extract.
Pour the wet ingredients into the dry ingredients and mix well.
Fold in the berries, if using.
Pour the mixture into a greased baking dish.
Bake for 35 minutes, or until the top is golden and the oatmeal is set.
Let it cool for a few minutes before serving.