Preheat the oven to 350 degrees F. Combine the first 5 ingredients in a bowl. Spoon the mixture into an 8" x 8" baking dish.
Combine the remaining ingredients, except butter, in another bowl. Add the butter and use your fingertips to mix and thoroughly blend it into the other ingredients. Sprinkle the oat mixture on the mango mixture. Bake until golden brown and bubbling, about 30 to 35 minutes.
Note: The mango in this recipe was cut into ½" cubes. If desired, you could top portions of this dessert with whipped cream or ice cream.