Crush the Lemon or Golden Oreos in a food processor until very fine; alternatively place cookies in a large resealable plastic bag and crush with a rolling pin.
Combine the crushed cookies with the melted butter in a bowl or bag and stir or shake until evenly moistened.
Press the buttered cookie crumbs firmly and evenly into the bottom of a 9x13-inch baking dish to form the crust.
In a medium bowl, beat the powdered sugar, softened cream cheese, and softened 1/2 cup butter with a hand mixer until smooth.
Fold in half of the whipped topping into the cream cheese mixture until combined, then spread this layer evenly over the crust.
In another bowl, whisk the two boxes of lemon instant pudding with 3 cups milk until the pudding thickens, about 1–2 minutes.
Pour the thickened lemon pudding over the cream cheese layer and spread gently to cover.
Top with the remaining whipped topping, spreading it into an even layer, then refrigerate for at least 1 hour (and up to 24 hours) before serving.
Garnish with lemon slices just before serving.