Preheat your oven to 375°F (190°C).
In a large bowl, mix together the oats, baking powder, cinnamon, and salt.
In another bowl, whisk together the milk, maple syrup, eggs, melted butter, and vanilla extract.
Pour the wet ingredients into the dry ingredients and mix until combined.
Fold in the berries or chopped fruit, if using.
Pour the mixture into a greased baking dish.
Bake for 35-40 minutes, or until the top is golden and the oatmeal is set.
Let cool slightly before serving.