Place cake mix, oil, eggs, and mandarin oranges in juice in a large bowl and beat until combined. Grease and flour 9 x 13 inch baking pan. Pour batter into pan and smooth out. Bake at 350F for 30 minutes. Let cake cool before frosting.
Combine dry pudding mix with drained pineapple. Fold in Cool Whip until well blended. Pour 3/4 cup pineapple juice over baked cake, spread frosting over cake. Keep in refrigerator.