Preheat the oven to 350°F (175°C) and lightly grease an 8×8-inch baking dish.
Spread the rolled oats evenly across the bottom of the baking dish.
Scatter the diced apples evenly over the oats.
In a mixing bowl, whisk together the milk, eggs, maple syrup, melted butter, cinnamon, baking powder, salt, and vanilla extract.
Pour the liquid mixture evenly over the oats and apples.
Sprinkle chopped nuts over the top if using.
Gently press the mixture down so the oats are fully submerged.
Bake uncovered for 35–40 minutes, until the center is set and the top is lightly golden.
Remove from the oven and let rest for 5–10 minutes before serving.