Preheat the oven to 350°F (177°C). Spray a 9-inch pie plate (or an 8 or 9-inch square baking dish) with nonstick cooking spray and set aside.
In a large mixing bowl, whisk together the flour, brown sugar, cinnamon, and salt until well combined.
Add the 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.
Add the sliced peaches to a large mixing bowl and sprinkle the flour and sugar on top. Using a spatula, mix it up until all of the peaches are fully coated.
Scoop the peaches into the prepared baking dish.
Sprinkle the topping on top of the peaches until it’s covers all the peaches.
Bake for 40 to 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.