Preheat oven to 325°F.

Crush Oreos into fine crumbs, mix with melted butter, and press into a 9-inch pie plate.

Bake for 12-15 minutes, then let cool.

In a microwave-safe bowl, heat chocolate, butter, oil, and cocoa powder in 20-second intervals, stirring until melted.

Mix in brown sugar, eggs, vanilla, and salt until smooth. Add flour and stir just until combined.

Pour into the cooled crust and bake for 25-30 minutes, until a toothpick comes out slightly wet. Cool completely.

Roughly crush Oreos in a ziplock bag, then mix with powdered sugar, cocoa powder, and salt.

Drizzle with melted butter and toss to coat.

Spread on a parchment-lined baking sheet and bake at 350°F for 10 minutes. Cool completely.

Melt chocolate in 20-second intervals in the microwave, stirring until smooth. Let cool slightly.

Whip heavy cream, cocoa powder, powdered sugar, and salt until soft peaks form.

Stir 1/3 of the whipped cream into the cooled chocolate, then fold in the rest until smooth.

Spread mousse over the cooled brownie layer. Cover and refrigerate at least 3 hours.

Whip heavy cream, sugar, and cocoa powder until stiff peaks form.

Pipe or spread whipped cream over the chilled pie, then top with cookie crumbs.