Crush the Oreo cookies into fine crumbs using a food processor or by placing them in a sealed bag and crushing with a rolling pin.
In a bowl, mix the Oreo crumbs with melted butter and sugar until the mixture resembles wet sand and holds together when pressed.
Press the crumb mixture firmly into the bottom of a springform pan and slightly up the sides to form an even crust. Place it in the refrigerator while preparing the filling.
In a large bowl, beat the softened cream cheese until smooth and creamy with no lumps.
Slowly pour in the sweetened condensed milk while mixing, and continue beating until the mixture is silky and well combined.
Add the hot cocoa mix one packet at a time, stirring well after each addition to ensure it fully dissolves into the mixture.
Gently fold in the whipped topping using a spatula, mixing just until combined to keep the texture light and airy.
Add the marshmallow fluff and lightly swirl it into the filling, leaving visible ribbons rather than fully mixing it in.
Pour the filling over the prepared crust and smooth the top evenly.
Freeze the cheesecake for at least 8 hours or overnight until firm.
Before serving, let it sit at room temperature for about 10 minutes to soften slightly for easier slicing.
Slice and serve with whipped cream if desired, then return any leftovers to the freezer.