In a large bowl combine orange Jello mix and boiling hot water. Use a whisk to stir until all Jello is dissolved. Add cold water and mix. Use a rubber spatula to scrape around the sides of the bowl. Cover bowl with plastic wrap and place in the refrigerator for 15-20 minutes.
Remove from the refrigerator and whisk in dry instant vanilla pudding mix. Use the rubber spatula to scrape sides of the bowl again to ensure all jello and pudding are dissolved. Cover bowl with plastic wrap and place back in the refrigerator for another 15-20 minutes.
Remove bowl from the refrigerator and gently fold in mandarin oranges, cool whip, and marshmallows. Cover bowl with plastic wrap and place back in the refrigerator for at least 1 hour, or until ready to serve.
It is best if the mandarin oranges are very well drained before adding to the fluff mixture.