Mix flour, oats, brown sugar, baking powder, and salt in a large bowl. Beat milk, peanut butter, and eggs in another bowl; stir into oat mixture, mixing until batter is well blended. Spoon batter evenly into 12 muffin cups.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 15 to 18 minutes.
