Preheat your oven to 325ºF (163ºC). In a food processor, pulse 16 Oreo cookies into fine crumbs. Combine the Oreo crumbs with melted butter and sugar, mixing until well incorporated. Press this mixture into the bottom and up the sides of a 9-inch pie plate. Bake for 15 minutes until set, then allow to cool completely.

In a heatproof bowl, melt bittersweet chocolate, butter, and vegetable oil over simmering water, stirring until smooth. Remove from heat, mix dark brown sugar, eggs, vanilla extract, cocoa powder, flour, and salt until just combined. Pour brownie batter into cooled Oreo crust and bake for 15 minutes. Cool at room temperature for 60 minutes, then chill in the refrigerator for at least 30 minutes.

Crush 6 additional Oreo cookies in a bowl. Mix crushed cookies with sugar, cocoa powder, and melted butter until it resembles damp sand. Spread this mixture on a baking sheet and bake at 325ºF (163ºC) for about 10 minutes. Remove from the oven and allow to cool completely.

Melt milk chocolate in a heatproof bowl until smooth, allow to cool slightly. In a separate bowl, whip heavy cream with cocoa powder, powdered sugar, and a pinch of salt until soft peaks form. Fold the whipped cream into the cooled melted chocolate. Spread the mousse evenly over the cooled brownie layer.

Sprinkle the prepared Oreo crumble topping evenly over the mousse. Cover with plastic wrap and refrigerate for at least 3-4 hours or ideally overnight.