Preheat the oven to 375 degrees F.
Add all of the apple ingredients to the apples and mix until well-coated. Set aside.
Combine the oats, flour, cinnamon, baking powder, salt, and brown sugar. Once combined, pour in the melted butter and use a fork to mix until combined and no dry flour remains.
Pour the apples into the baking dish and spread them out evenly. Top with the oat topping so that it covers the apples and bake in the preheated oven for 40 minutes, or until the top is crispy and it's slightly bubbling.
Let rest for 20 minutes before cutting. Serve with whipped cream or vanilla ice cream, if desired.