Mix together the flour, baking soda and salt in bowl, set aside.
Using a stand mixer, or large bowl with a hand mixer, cream the butter, sugar and brown sugar until fluffy. Add the vanilla and mix.
Add eggs, one at a time, beating after each egg.
Reduce speed, and gradually mix in the flour mixture.
Stir in chocolate chips.
Mix the brownie batter together following the instructions on the box.
Grease a 9x13 pan with melted butter or cooking spray. Press the cookie dough into the bottom of the pan, then add the Oreos over the top (as many as you can fit!). Pour the brownie batter over the top and smooth so it is evenly spread
Bake at 350 for 35 -45 minutes (until a toothpick comes out clean).