Preheat oven to 325°F. Finely crush 25 Oreos and mix with 6 tablespoons melted butter. Press into a 9-inch pie plate and bake for 12-15 minutes until set.

Melt bittersweet chocolate, 3 tablespoons butter, oil, and 2 tablespoons cocoa powder in the microwave. Whisk in brown sugar, eggs, vanilla, and salt, then stir in flour. Pour into the crust and bake for 25-30 minutes until slightly wet in the center.

Prepare the topping by crushing 8 Oreos and tossing with powdered sugar, 1 tablespoon cocoa powder, salt, and 2 tablespoons melted butter. Bake on a parchment-lined sheet for 10 minutes and cool.

For the mousse, melt milk chocolate and let cool. Whip 1 cup heavy cream with 2 tablespoons cocoa powder, 2 tablespoons powdered sugar, and salt to soft peaks. Fold the whipped cream into the melted chocolate.

Spread the mousse over the cooled brownie layer and refrigerate for at least 3 hours or overnight.

Whip the remaining heavy cream with granulated sugar and 3 tablespoons cocoa powder to stiff peaks. Spread over the chilled pie and sprinkle with the prepared cookie topping.