Preheat oven to 350 degrees Fahrenheit and spray a 9x13" pan lightly with nonstick cooking spray.
In a large bowl, add 2 1/2 c. flour, oats, brown sugar, softened butter and salt and mix with your hands or with a fork until the butter is incorporated and no loose flour or ingredients remain. Small crumbs will form.
Reserve 1 1/2 c. of the crumb mixture for the topping and set aside.
Press the remaining crumb mixture into the bottom of the prepared pan. Bake for 10-12 minutes.
While the crust is baking, make the filling mixture by stirring together 4 c. sliced peaches, 3 Tbsp. flour, 1/2 c. granulated sugar and 1/2 tsp. cinnamon.
After 10-12 minutes, remove the crust from the oven and spread the peach filling mixture evenly over.
Sprinkle with the reserved crumb mixture for the topping.
Return the pan to the oven and bake for approximately 20-25 more minutes, or just until the topping looks done and the peaches are bubbly. Allow it to cool some, then slice into bars and serve with a big scoop of vanilla ice cream on top! We love them warm the best!