Heat oven to 350°F. Spray 13x9-inch pan with nonstick cooking spray. In large bowl, combine brown sugar and butter; beat until smooth. Stir in oats and cereal. Add flour, baking soda and salt; mix until well blended. Reserve 3 cups oat mixture for topping. Press remaining mixture in bottom of sprayed pan. Bake at 350°F. for 10 minutes.