Place the orange gelatin in a large mixing bowl. Pour in the boiling water and stir until every grain of gelatin has dissolved, scraping the bottom and sides of the bowl as needed.

Stir in the cold water until the gelatin mixture is evenly combined. This cools the base and prepares it for the fruit without making the whipped topping warm later.

Add the undrained crushed pineapple and well-drained mandarin orange segments. If using cottage cheese, stir it in now, taking care not to crush all the mandarin pieces.

Refrigerate the mixture for about 30–45 minutes, until it has cooled and begun to thicken but is not fully set. Check it with a spoon; it should look slightly syrupy and thick rather than firm.

Fold in the thawed whipped topping and miniature marshmallows with a gentle hand. Add the chopped pecans if using, folding just until everything is distributed and the mixture remains fluffy.

Transfer the orange fluff to a serving bowl, cover it, and refrigerate for at least 2 hours, preferably longer. Chilling overnight gives the dessert its best softly set texture and allows the flavors to mingle.

Garnish just before serving with extra mandarin orange segments, a little whipped topping, or chopped pecans. Spoon the chilled fluff into individual dishes or serve it family-style from the bowl.