Preheat your oven to 350°F (175°C). Lightly grease a baking dish to prevent sticking.
Slice the peaches into thin, even pieces and place them in a large mixing bowl. Add granulated sugar and ground cinnamon, then gently toss to coat all the slices evenly.
Transfer the sugared peach slices into your prepared baking dish, spreading them out into a single, even layer.
In a separate bowl, combine rolled oats, flour, and brown sugar. Mix together until well blended.
Add the cold, diced butter to the dry mixture. Using a pastry cutter or fork, work the butter into the oat mixture until it resembles coarse crumbs with some pea-sized bits. This creates the crisp topping.
Sprinkle the crumbly topping evenly over the peaches in the baking dish, covering all the fruit for a uniform crust.
Place the baking dish in the preheated oven and bake for about 40 minutes, or until the topping is golden brown and the peach mixture is bubbly around the edges.
Remove the peach crisp from the oven and let it cool slightly for 10 minutes. The aroma of baked cinnamon and peaches will fill the kitchen.
Slice and serve the warm peach crisp with a scoop of vanilla ice cream or a dollop of whipped cream, if desired.
