Heat oven to 350°F. Spray 15x10x1-inch baking pan with nonstick cooking spray.* Place melted butter in large bowl. Reserve 3 tablespoons of the cake mix; set aside. Add remaining cake mix to butter; blend well. Add egg; mix well. Stir in oats. Press 2/3 of cake mix mixture (about 2 1/2 cups) in bottom of sprayed pan.