Preheat your oven to 325°F (163°C). In a food processor, blend the Oreos into fine crumbs, then mix with melted butter and sugar. Press into a 9-inch pie plate. Bake for 15 minutes and let cool.

In a microwave-safe bowl, combine bittersweet chocolate, butter, vegetable oil, and cocoa. Heat until melted and smooth. In another bowl, whisk together brown sugar, eggs, vanilla, and salt. Stir in the chocolate mixture, then add flour. Pour into cooled crust and bake for 15 minutes; allow to cool for 60 minutes before refrigerating.

Crush the Oreos until fine, then mix with powdered sugar, cocoa, salt, and melted butter. Spread on a baking sheet and bake for 10 minutes. Let cool.

Melt the milk chocolate and let cool. Heat 3 tablespoons of cream with cocoa and sugar. Whip remaining cream to soft peaks, fold in chocolate. Spread mousse over cooled brownie layer, top with the topping mixture. Refrigerate for 3-4 hours or overnight.