Mix oats, 1/2 cup sugar, and flour in bowl. Add butter; rub in with fingertips until topping comes together in moist clumps. (Can be made 1 day ahead. Cover; chill.)
Preheat oven to 375°F. Spray 13x9x2-inch glass baking dish with nonstick spray. Mix apples, lemon juice, cinnamon, and 1/4 cup brown sugar in bowl. Transfer to dish. Sprinkle topping over.
Bake crumble until apples are tender and topping is brown and crisp, about 55 minutes. Cool slightly. Spoon warm crumble into bowls. Serve with ice cream.
Bon Appétit, October 2003, Entertaining with Style