Preheat the oven to 350 degrees and line an 8-inch square pan with parchment paper.
In a bowl, toss the diced apples with 1 teaspoon cinnamon until evenly coated.
In a separate bowl, stir together the flour, oats, brown sugar, granulated sugar, remaining teaspoon of cinnamon, and salt.
Pour the melted butter into the flour mixture and stir until large crumbs form.
Press half the crumble mixture into the bottom of the prepared pan to form a base.
Spread the cinnamon apples evenly over the base.
Sprinkle the remaining crumble mixture over the apples and press down lightly.
Bake for 30 minutes until the top turns golden.
Cool completely in the pan before cutting into bars.