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.
Notes
Oats: I prefer old-fashioned oats (or sprouted old-fashioned oats) for this recipe because we like the soft granola-ish texture. For a softer texture, quick oats will work. Milk: feel free to increase the milk if you like softer/plumper oatmeal. Butter vs Oil: I prefer using melted butter for flavor if baking the oatmeal right away; however, if I'm making it the night before and baking the next morning, I prefer using oil so the butter doesn't clump in the refrigerator (coconut oil will do the same thing). Overnight: if refrigerating the pan of oatmeal overnight, before baking in the morning, give the mixture a quick stir right in the pan. Sweeteners: you can adapt and alter the amount and type of sugar/syrup in the recipe according to your tastes.