Gather all ingredients.
Prep the apples – Peel, core, and slice the apples into thin pieces. Toss with lemon juice, sugar, cinnamon, nutmeg, and cornstarch.
Make the oat topping – In a bowl, mix oats, flour, brown sugar, salt, and cinnamon. Cut in the butter until crumbly.
Assemble – Spread apple mixture in a greased 9x13-inch baking dish. Sprinkle oat topping over apples.
Bake – Bake at 350°F (175°C) for 40–45 minutes, until topping is golden brown and apples are bubbling.
Serve – Let cool slightly, then serve warm with vanilla ice cream or whipped cream.