Whisk together the Oreo pudding mix and milk until well combined. Allow to rest until the pudding begins to thicken, around 5 minutes
Gently fold in the whipped topping until well mixed.
Add the mini marshmallows and crushed cookies and stir to mix.
Cover and chill for at least one hour before serving. Top with extra Oreos, if desired.
Notes
Storage: Cover leftovers with plastic wrap (or put it in an airtight container) and keep it up in the refrigerator for up to three days. When serving leftovers, gently stir the fluff before serving again.