Preheat your oven to 350 degrees Fahrenheit and line a 9x9 inch baking pan with parchment paper.
In a large bowl, combine the flour, oats, brown sugar, baking soda, and salt.
Stir in the melted butter until the mixture is crumbly.
Press two-thirds of the oat mixture firmly into the bottom of the prepared pan.
In a small bowl, toss the diced mango with lemon juice and cornstarch.
Spread the mango mixture evenly over the crust.
Sprinkle the remaining oat mixture over the top of the mangoes.
Bake for 35 to 40 minutes until the top is golden brown. Let cool completely before cutting into bars.