In a large saucepan, combine the orange gelatin and vanilla pudding mix. Add hot water and stir over medium heat until dissolved, about 5 minutes.

Increase heat to bring the mixture to a gentle boil, stirring frequently until it thickens and turns clear, about 2-3 additional minutes.

Pour the hot mixture into a large mixing bowl. Cool at room temperature for 30 minutes, then refrigerate until set, about 4 hours or overnight.

Once set, use a potato masher to break apart the jello and mix in the Cool Whip until combined.

Gently fold in the drained mandarin oranges.

Transfer to a serving dish and chill for at least another hour before serving.