In a medium bowl, whisk together the flour, oats, sugar, and cinnamon. Use your fingertips to pinch the cold butter into the oat mixture until you get pea-size crumbles. Work quickly so the butter doesn't get too warm.
Using a paring knife or an apple core, remove most of the center core, leaving about 1/2-inch at the bottom. You should end up with a hole for the filling.
Place the apples into an air fryer. Carefully add the oat mixture to the cavity in each apple until full.
Turn the air fryer to 350 degrees F and cook for 20 minutes.
Serve warm topped with vanilla ice cream and caramel sauce, if desired.
Notes
Storage & Reheating:You can store any leftovers in an airtight container in the refrigerator for up to 4 days. If you’d like to reheat them, pop an apple in the microwave for 30 seconds, or in the air fryer for 4 to 5 minutes at 350 degrees F.