Lightly coat a 9-inch springform pan with nonstick spray and set aside.
Place the 20 Oreos in a zip-top bag and crush with a rolling pin, or pulse in a food processor until fine crumbs form.
Combine the cookie crumbs and 6 tablespoons melted butter in a bowl and stir until evenly moistened.
Press the crumb mixture into the bottom of the prepared pan and up the sides about 1 inch using a measuring cup or spoon; refrigerate at least 20 minutes.
Pour 2/3 cup cold water into a small bowl, sprinkle the unflavored gelatin (4 envelopes) over the water and let it bloom 1–2 minutes, then microwave for 30 seconds and whisk until smooth; set aside to cool slightly.
Place the 12 ounces chopped milk chocolate in a microwave-safe bowl and heat in 30-second intervals, stirring between, until fully melted and smooth; set aside to cool slightly.
In a large bowl, beat the 32 ounces softened cream cheese until smooth, then add 1 1/2 cups Nutella and the melted chocolate and beat until combined, scraping the bowl as needed.
In a separate bowl, whisk the 2 cups cold heavy whipping cream by hand, then add the cooled gelatin mixture and whisk to combine.
Pour the cream-and-gelatin mixture into the cream cheese mixture and use an electric mixer to beat until fully combined and smooth.
Spread the cheesecake filling evenly into the chilled crust, then refrigerate for 5–6 hours or overnight until firm.
For the topping, whip 2 cups heavy cream with 1/4 cup sugar and 1 teaspoon vanilla until stiff peaks form.
Transfer the whipped cream to a piping bag fitted with a star tip and pipe decorative dollops around the cake edge, then garnish with mini Oreos.
Run a butter knife around the pan edge to loosen, remove the springform ring, and slice the cheesecake, wiping the knife between cuts for clean slices.