Preheat the oven: Preheat your oven to 350°F (177°C). Grease a 9-inch round or square baking dish with nonstick spray and set aside.
Make the crisp topping: In a large mixing bowl, whisk together the flour, brown sugar, cinnamon, and salt. Add the cold, cubed butter and cut it into the dry ingredients using a pastry cutter or fork until the mixture is crumbly. Stir in the rolled oats. Chill the topping in the refrigerator while you prepare the filling.
Prepare the peach filling: In a separate large bowl, combine the sliced peaches, granulated sugar, and flour. Toss until the peaches are evenly coated.
Assemble the crisp: Pour the peach mixture into the prepared baking dish and spread evenly. Sprinkle the chilled oat topping evenly over the peaches.
Bake: Bake for 40 to 50 minutes, or until the top is golden brown and the peach filling is bubbling around the edges.
Cool and serve: Allow the crisp to cool slightly before serving. Enjoy warm with vanilla ice cream or whipped cream for an extra treat.