In a stand mixer with a whisk attachment (or in a large mixing bowl with a hand mixer) beat together the milk and pudding mix until thickened, about 3 minutes.
Add the cream cheese to the bowl and mix until smooth.
Fold in all of the Cool Whip using a rubber spatula. Then fold the chopped Oreos and mini marshmallows into the cream cheese mixture.
Cover the bowl with plastic and refrigerate for at least 1 hour before serving. (As the mixture sits, the oreo pieces absorb the moisture and expand and biting into one is like getting a little piece of cake.)
Notes
Marshmallows can be increased up to 3 cups if desired.
This fluff will last for up to 2 days in the fridge stored in an air-tight container.