Preheat your oven to 325°F (163°C). Process 16 Oreo cookies into fine crumbs. In a mixing bowl, combine the cookie crumbs with melted butter and press into a 9-inch pie plate. Bake for 15 minutes and let cool completely.

Melt 6 ounces of bittersweet chocolate, 1/2 cup butter, 1/4 cup vegetable oil, and 1/3 cup cocoa powder in the microwave. In a separate bowl, whisk together 2 eggs, 1 cup dark brown sugar, 1 teaspoon vanilla extract, and a pinch of salt. Combine both mixtures and fold in 1/2 cup flour. Pour over the cooled crust and bake for 15 minutes.

Crush 6 Oreo cookies and mix with 1 tablespoon cocoa powder, 2 tablespoons sugar, and 3 tablespoons melted butter. Spread on a baking sheet and bake for 10 minutes. Let cool.

Melt 4 ounces of milk chocolate. In a bowl, whip 1 cup heavy cream with 2 tablespoons cocoa powder and powdered sugar until soft peaks form. Fold in melted chocolate and spread over the cooled brownie layer.

Sprinkle the crumb topping over the mousse layer. Cover with plastic wrap and refrigerate for at least 3-4 hours or overnight.