Preheat oven: Set oven to 350°F (175°C). Grease a 9x13-inch baking dish or similar size.
Prepare the apples: Peel, core, and slice apples into ¼-inch thick slices. Place in a mixing bowl and toss with lemon juice, sugar, cinnamon, nutmeg, and flour. Spread evenly into the prepared baking dish.
Make the crumb topping: In another bowl, combine oats, flour, brown sugar, cinnamon, and salt. Add cold butter cubes. Using a pastry cutter or fingertips, cut butter into the mixture until it resembles coarse crumbs.
Assemble the crisp: Sprinkle the oatmeal crumb topping evenly over the apples.
Bake: Bake for 45–50 minutes, until apples are bubbling and topping is golden brown.
Cool slightly and serve: Let rest for 10 minutes before serving to allow juices to set.