For the topping:

1 cup all-purpose flour
1.5 cup brown sugar
1 teaspoon ground cinnamon
1/2 teaspoon salt
1 cup butter
2 cup old-fashioned rolled oats

For the filling:

5 cups sliced peaches, about 6-7 medium peaches
(optional) 1 cup blueberries
1/3 cup granulated sugar
1/4 cup all-purpose flour 

Preheat the oven to 350°F.
To make the topping: In a mixing bowl, whisk together the flour, brown sugar, cinnamon, and salt until well combined. Add the cold cubed butter and using a pastry cutter or fork, cut the butter into the dry ingredients until the mixture starts to come together and is crumbly. Stir in the oats and transfer to the refrigerator while you make the filling.
To make the filling: Add the peaches to a large mixing bowl and sprinkle the flour and sugar on top. Stir until all of the peaches are well coated with the flour and sugar. Scoop the peaches into a baking dish, remove the topping from the refrigerator and sprinkle on top of the filling.
Bake at 350°F for 40-50 minutes or until the topping is lightly golden brown and the juices are bubbling around the edges.Remove from the oven and transfer to a wire rack to cool slightly before serving.