In a bowl, combine the oats, flour, brown sugar, baking soda and salt. Add butter; mix until crumbly. Set aside 1 cup. Press the remaining crumb mixture into a greased 13-in. x 9-in. baking pan. Bake at 350° for 9-11 minutes or until edges are lightly browned (bars will puff up slightly).
Meanwhile, set aside 1 cup M&M's; chop remaining M&M's. In a bowl, combine the milk, peanut butter and vanilla; mix well. Stir in chopped M&M's. Pour over crust; carefully spread evenly. Sprinkle with reserved M&M's; gently press into peanut butter mixture. Sprinkle with reserved crumb mixture. Bake 18-22 minutes longer or until edges are lightly browned. Cool on a wire rack before cutting.