HEAT oven to 350ºF.
COMBINE oats, flour and baking soda. Beat butter and sugar in large bowl with mixer until blended. Add oat mixture; mix well. Press 3 cups onto bottom of 13x9-inch pan sprayed with cooking spray.
MICROWAVE chocolate in small microwaveable bowl on HIGH 1-1/2 min. or until melted, stirring after 45 sec. Stir in peanut butter. Carefully spread over crust; sprinkle with remaining crumb mixture.
BAKE 22 to 25 min. or until edges are golden brown.