Prepare the Crust:

1. Preheat the oven to 325°F (163°C).
2. In a food processor, pulse 16 Oreo cookies into fine crumbs.
3. Add 1 tablespoon granulated sugar and 4 tablespoons melted butter; pulse until combined.
4. Press the mixture firmly into the bottom and up the sides of a 9-inch pie plate.
5. Bake for 15 minutes until fragrant. Cool on a wire rack.

Prepare the Brownie Layer:

1. In a microwave-safe bowl, combine 4 ounces chopped bittersweet chocolate, 3 tablespoons unsalted butter, 3 tablespoons vegetable oil, and 1 ½ tablespoons cocoa powder. Microwave at 50% power, stirring frequently, until melted and smooth (about 1 minute 30 seconds).
2. In another bowl, whisk together ⅔ cup dark brown sugar, 2 large eggs, 2 teaspoons vanilla extract, and ¼ teaspoon salt until smooth.
3. Add the melted chocolate mixture and whisk until combined.
4. Stir in 3 tablespoons all-purpose flour until just incorporated.
5. Pour the brownie batter into the cooled Oreo crust.
6. Bake at 325°F (163°C) for about 15 minutes, until the edges are set and a toothpick inserted into the center comes out with a thin coating of batter.
7. Cool on a wire rack for 1 hour, then refrigerate for at least 1 hour until completely chilled.

Prepare the Topping:

1. In a food processor, pulse 6 Oreo cookies into coarse crumbs.
2. Add 2 tablespoons powdered sugar, 1 tablespoon cocoa powder, and ⅛ teaspoon salt; pulse to combine.
3. Drizzle in 2 tablespoons melted butter and pulse until the mixture resembles wet sand.
4. Spread the crumb mixture evenly on a baking sheet.
5. Bake at 325°F (163°C) for about 10 minutes until fragrant. Cool completely.

Prepare the Mousse:

1. Melt 6 ounces milk chocolate in a microwave-safe bowl at 50% power, stirring frequently, until smooth. Let cool until the chocolate reaches 85°F (29°C).
2. In a chilled mixing bowl, whisk ½ cup heavy cream until soft peaks form; refrigerate.
3. In another bowl, whisk the remaining ½ cup heavy cream, 2 tablespoons cocoa powder, 2 tablespoons powdered sugar, and a pinch of salt until thickened.
4. Fold the cocoa mixture into the melted chocolate until combined.
5. Gently fold in the whipped cream until no streaks remain.

Assemble the Pie:

1. Spread the chocolate mousse evenly over the chilled brownie layer.
2. Sprinkle the cooled cookie crumble topping over the mousse layer.
3. Refrigerate the pie for at least 4 hours until set.