Preheat oven to 350 degrees. Line a 9-inch square pan with foil, allowing foil to hang over sides of pan for easy removal after baking. Spray with cooking spray.
Make brownie batter as directed on box. Spread in prepared pan. Bake 32 to 35 minutes or until brownies are set around sides and toothpick inserted in center of brownies comes out almost clean. Cool in pan 10 minutes.
Meanwhile, in medium bowl, make cookie dough as directed on pouch for drop cookies. Stir in 1 cup of the M&M’s candies. Drop half of the cookie dough mixture in chunks on top of baked brownie layer; add half of the Oreo pieces.
Repeat with remaining cookie dough and Oreo pieces. With back of spoon or offset metal spatula, gently press cookie dough together to form even layer. Top with remaining 1/4 cup candies, pressing them gently into the cookie dough.
Bake 23 to 28 minutes or until cookie layer is set, covering with foil the last 10 minutes of baking if necessary to prevent overbrowning. Cool at least 2 1/2 hours; remove bars from pan, and cut into 4 rows x 4 rows.