Preheat your oven to 350°F (175°C) and lightly grease an 8x8-inch baking dish.
In a large bowl, combine the rolled oats, baking powder, cinnamon, and salt. Mix well.
In another bowl, whisk together the milk, maple syrup (or honey), eggs, and vanilla extract until fully combined.
Pour the wet ingredients into the dry ingredients and stir until just combined.
Gently fold in the mixed berries and chopped nuts if using.
Pour the mixture into the prepared baking dish and spread it evenly.
Bake for 25-30 minutes, or until the top is golden brown and the oatmeal is set.
Allow to cool for a few minutes before slicing into squares and serving.