Brush a 10-inch cast-iron skillet with oil, and heat in a 425° oven 5 minutes.
Stir together cornmeal and next 3 ingredients in a large mixing bowl. Stir in buttermilk, eggs, and mayonnaise, stirring just until blended. Pour batter into hot skillet.
Bake at 425° for 18 to 20 minutes or until golden brown and cornbread pulls away from sides of skillet. Remove to a wire rack, and cool 5 minutes.