Preheat the oven to 350°F. Grease a 9x9-inch baking dish or line it with parchment paper.
In a large bowl, combine the rolled oats, flour, brown sugar, salt, and cinnamon. Pour in the melted butter and mix until the mixture resembles coarse crumbs. Reserve 1 cup of this mixture for the topping.
In another bowl, combine the diced mango, granulated sugar, lemon juice, and cornstarch. Toss gently to coat the mango evenly.
Spread the remaining oat mixture evenly in the bottom of the prepared baking dish. Pour the mango mixture over the crust, spreading it out evenly.
Sprinkle the reserved oat mixture on top of the mango layer.
Bake in the preheated oven for 30-35 minutes, or until the topping is golden brown.
Remove from the oven and let cool in the pan for at least 15 minutes before cutting into bars.