In a large bowl, whisk the instant vanilla pudding mix with chilled milk until smooth and thickened.
Fold in the sweetened condensed milk and mix until combined.
Gently fold in the whipped topping until well mixed.
Add the crushed Oreo cookies and mini marshmallows to the mixture, folding them in until evenly distributed.
Refrigerate for at least 1 hour before serving to allow the flavors to meld.
Notes
This easy Oreo fluff can be made a day ahead and stored in the refrigerator. Decorate with additional crushed Oreos or mini marshmallows before serving if desired.