Preheat oven to 350°F.
Peel, core, and slice the apples.
Place apples in a large bowl and toss with granulated sugar, flour, cinnamon, and lemon juice.
Transfer the apple mixture to a baking dish and spread evenly.
In a separate bowl, combine rolled oats, flour, brown sugar, and cinnamon.
Add melted butter and stir until a crumbly topping forms.
Sprinkle the crumb topping evenly over the apples.
Bake for 45 minutes, or until the apples are tender, the filling is bubbling, and the topping is golden brown.
Allow the apple crisp to cool for several minutes before serving.
Serve warm on its own or with vanilla ice cream.