Preheat oven to 375°F. Prepare a 9x13-inch baking dish with nonstick cooking spray or butter.

In a large bowl, combine sliced apples, 0.75 cup brown sugar, 1 tablespoon flour, and cinnamon. Toss to coat evenly and set aside.

In another bowl, mix oats, 2 cups flour, 1.5 cups brown sugar, and a pinch of cinnamon to make the crisp topping.

Cut in the cold butter using a fork or pastry cutter until the mixture resembles pea-sized crumbs.

Spread the apple mixture evenly into the prepared baking dish. Sprinkle the topping evenly over the apples.

Bake for 40 to 45 minutes, or until the topping is golden brown and the apples are bubbling.

Serve warm, optionally with vanilla ice cream or whipped cream.