Preheat the oven to 375 degrees. Prepare an 8×8 baking dish by spraying it with cooking spray or lining it with parchment paper.
In a large mixing bowl, mix together the oatmeal, brown sugar, baking powder, cinnamon, and salt
Stir the melted butter into the dry ingredients until well mixed.
Add in the vanilla and milk and stir to mix well.
Add in the diced peaches and mix until they are well distributed.
Pour the batter into the prepared baking dish and smooth out evenly. Arrange the peach slices evenly across the top of the oatmeal.
Bake the oatmeal for 45 to 50 minutes.
Let the baked oatmeal cool completely for about 10 minutes before cutting and serving.
Serve topped with maple syrup, whipped cream, or your favorites oatmeal toppings.