Chop the apples into roughly ½-inch pieces, peeling is optional.
In a bowl, combine the apples, sugar, corn starch, cinnamon, nutmeg, and lemon zest, and stir until fully coated.
In a second bowl, whisk together the oats, flour, sugar, cinnamon, nutmeg, and salt.
Drizzle the melted butter over the dry topping mixture and stir with a fork until crumbly, with small clumps forming.
Divide the apple filling among 4 ramekins or transfer to one 6-inch baking dish, pressing down gently to compact.
Scatter the oat topping evenly over the apples without pressing it down.
Place in the air fryer basket and cook at 375°F for 12 to 15 minutes, until the topping is golden and the filling is bubbling at the edges.
Test the apples with a fork for tenderness, adding 2 to 3 more minutes if needed.
Let cool for 5 minutes before serving.