Preheat your air fryer to 390℉ (if your model has this option). Lightly grease 4 small ramekins or an 8x8 baking dish (use a dish that will fit into your air fryer). Set aside.
In a medium bowl, add the chopped apples, brown sugar, cinnamon, nutmeg, and salt. Stir well to combine. Spoon mixture evenly into prepared ramekins or dish.
Cover ramekins/dish with foil and place into the air fryer basket; making sure the foil is tucked underneath the bottom of dish(es) to prevent it from flying around the air fryer and possibly burning. Cook for 8-10 minutes, then remove the foil and check to make sure the apples are tender. Then add the topping as listed below.
While apples are in the air fryer, make the topping: in another medium bowl, add the oats, flour, brown sugar, melted butter, and cinnamon. Stir until mixture becomes crumbly.
Spoon topping evenly between the ramekins and place back into the air fryer, uncovered. Cook for another 5-7 minutes, keeping a careful eye on it, as it's easy to burn the topping in the air fryer, or until topping is crisp. Remove from air fryer and let cool for several minutes, then serve with vanilla ice cream, if desired.