
Preheat the oven to 350 degrees F. (Skip this step right now if making the oatmeal overnight.)
Lightly grease a 9X13-inch baking pan (or smaller pan for thicker oatmeal). Set aside.
In a large bowl, add the oats, brown sugar, baking powder, and salt. Stir to combine.
In another bowl or liquid measuring cup, whisk together the milk, maple syrup, eggs, vanilla, and butter or oil.
Add the wet ingredients to the oats mixture and stir to combine.
Pour the mixture into the prepared pan. Cover and refrigerate until ready to bake (up to 24 hours) or bake right away.
Bake uncovered for 25-30 minutes until the edges are lightly golden and the oatmeal is set. Serve warm with additional toppings.