Use 1 teaspoon of butter to grease 2 ramekins that will fit into your air fryer.
Combine apples, water, sugar, cinnamon, and salt in a small bowl. Spoon into the prepared ramekins and cover with aluminum foil.
Air fry at 350°F (175°C) for 15 minutes.
Mix together the rolled oats, flour, brown sugar, and melted butter in a small bowl until it resembles rough gravel.
When the 15 minutes is up, remove the foil from the top of the ramekins. Spoon the topping onto the top of the apple mixture in each ramekin. Air fry for another 6 minutes to crisp up the topping.