Preheat oven to 350 F. Line a rimmed baking sheet with parchment paper or a silicone baking mat for easier clean up. Set aside.
Core each apple without cutting all the way through and make a good-sized in the well center of each apple.
4 baking apples
In a medium bowl, stir oats, brown sugar, and apple pie spice together.
1 cup rolled oats, ¼ cup light brown sugar, 1 teaspoon apple pie spice
Stir in butter until combined.
¼ cup salted butter
Spoon 1/4 of the oat mixture into each apple, slightly mounding the filling.
Place apples on prepared rimmed baking sheet.
Bake until apples are tender and filling is lightly browned, about 30 minutes.
Notes
Apples: Some types of baking apples that work well are Granny Smith, Jonagold, and Honeycrisp.
Apple pie spice: You can use ground cinnamon instead.
Baking time: The baking time is for a medium, standard-sized apple. If using a large, or extra-large apple, it will be necessary to add additional baking time.
Add-ins: Feel free to customize the filling by adding chopped nuts or raisins.