Preheat the oven to 350 degrees.
Slice your peaches and place in the bottom of a 9x13 baking dish. If using canned peaches, make sure to drain them first.
Combine flour, brown sugar, and oats in a large mixing bowl. Cut the butter in until the mixture is crumbly. It's easiest to do when the butter isn't right out of the refrigerator, but when it's had a few minutes on the counter--but not at room temperature or softened.
Cover the peaches with the crumbly topping, then bake for 45 minutes or until golden brown and bubbly.