Preheat oven to 350°F. Grease a 13x9-inch cake pan.
In a large bowl, combine the cake mix, eggs, oil, and mandarin oranges (including the juice). Beat for 2 minutes on medium speed with a hand mixer.
Pour batter into the prepared pan and bake for 30–35 minutes, or until a toothpick inserted into the center comes out clean. Let cool completely.
When the cake is cool, place the (drained) crushed pineapple and instant pudding in a large bowl and stir to combine. Fold in the defrosted Cool Whip.
Frost the cake and refrigerate until ready to serve.
Notes
Store the cake in the refrigerator. You can also freeze the cake for up to 3 months.You can substitute 1.5 cups of heavy cream, whipped to stiff peaks with 1–2 tablespoons of powdered sugar, for the Cool Whip.If you'd like to garnish the cake, place a mandarin orange segment (from another can) on top of each slice. I think it looks really pretty this way.