Line a 9 x 9-inch baking pan with parchment paper allowing the ends to extend over two of the sides, spray with nonstick spray, and set aside. Preheat the oven to 375°F (190°C) and position an oven rack at the top of the oven.
In a large bowl, stir together the 2 cups old fashioned rolled oats, 1 cup all-purpose flour, ½ cup granulated white sugar, ½ cup brown sugar, ½ cup sliced almonds, 1 teaspoon baking powder, 1 teaspoon ground cinnamon, and ½ teaspoon kosher salt. Add the melted 1 cup unsalted butter and 1 teaspoon almond extract. Stir until crumbly.
Press half of the dough into the bottom of your baking pan and bake for 10-12 minutes or until lightly golden brown on the edges.
While the crust is baking, chop the 4 large peaches into 1-inch pieces. In a large bowl coat the peaches with the 1 tablespoon cornstarch. Layer the peaches over the baked crust and top with the remaining crust mixture.
Bake for 30-40 minutes until the crust is light golden brown.
Cool completely then refrigerate for one hour. Use the parchment paper to lift the bars from the pan and cut them into squares.