In a large bowl, whisk together the instant pudding mix and milk until the mixture thickens slightly.
Add the softened cream cheese to the bowl and whisk until the mixture is smooth and lump-free.
Gently fold in the whipped topping and then mix in the crushed Oreo cookies and marshmallows (if using) until all ingredients are evenly combined. Cover the bowl and refrigerate for at least 2 hours before serving.
Garnish with additional whole or crushed Oreos just before serving.