Preheat an air fryer to 350 degrees F (175 degrees C).
Combine apples, brown sugar, butter, vanilla, cinnamon, nutmeg, and salt in a bowl. Toss and stir until apples are coated.
Spray 4 individual-sized ramekins with nonstick cooking spray. Divide apple mixture evenly among all 4 ramekins. Cover with foil and air fry until apples are tender, about 20 minutes.
Meanwhile, make streusel in the same bowl by combining oats, flour, brown sugar, butter, cinnamon, and salt. Use your hands to combine streusel. Place in the refrigerator until ready to use.
Remove ramekins from the air fryer, divide streusel evenly on top of the apples, and air fry, uncovered, until streusel is a golden brown, 5 to 7 minutes.