Grease a 9-inch springform pan and line the bottom with parchment paper.
Crush Oreos into fine crumbs and mix with melted butter. Press the mixture into the pan and refrigerate.
Microwave chocolate in 30-second bursts, stirring each time, until smooth.
Chill a bowl and whisk, then beat heavy cream until soft peaks form. Add powdered sugar and continue beating until stiff peaks form. Set aside.
Beat cream cheese until smooth, then add sugar, cocoa powder, and vanilla. Beat until smooth.
Add melted chocolate to the cream cheese mixture and mix well.
Gently fold in whipped cream and stir in crushed Oreos. Pour into the crust and refrigerate for 3-4 hours.
Make whipped cream for the topping, pipe it around the edges, and garnish with more crushed Oreos and hot fudge.