In the bowl of a food processor, pulse Oreo cookies until they are fine crumbs. Add the melted butter and pulse a few times to combine.

Press the crumb mixture into the bottom of a 9x13-inch baking dish.

In a medium bowl, beat the cream cheese with the sugar and milk with a hand mixer until the mixture is light and fluffy. Gently fold in 8 oz. of the Cool Whip with a rubber spatula until the mixture is well combined. 

Spread the cream cheese mixture evenly over the top of the crumb mixture. Refrigerate while preparing the next layer.

In a separate medium bowl, beat the pistachio pudding mix with the 2-1/2 cup of milk with a hand mixer until the mixture begins to thicken slightly. 

Carefully spread the pudding mixture evenly over the top of the cream cheese layer. 

Next, spread the remaining 8 oz. of Cool Whip evenly over the top of the pudding mixture. Sprinkle the chopped pistachios over the top. 

Cover and refrigerate for at least 4 hours before serving.