Preheat oven to 350°F (175°C). Lightly butter a 3-quart baking dish.
In a medium bowl, whisk together rolled oats, oat flour, nuts, brown sugar, cinnamon, and salt. Add melted butter, stirring until mixture is crumbly. Refrigerate until firm (about 15 minutes.)
In a large bowl, toss apples with lemon juice, brown sugar, cornstarch, cinnamon and nutmeg. Pour into prepared baking dish. Sprinkle the topping over the apples.
Bake for 30 to 35 minutes, or until filling is bubbly, apples are tender and topping is golden brown. Let crisp rest for at least 15 minutes before serving.