Preheat oven to 375 degrees. Cut apples in half lengthwise. Use a melon baller or small spoon to scoop out the core, making a 1-inch well in the middle of each apple half. Set apples in a baking dish cut side up. Combine dry ingredients in a medium bowl. Add butter and use a pastry cutter or your hands to mix butter into dry ingredients until mostly combined. Pack each apple half with a generous mound of oat mixture. Cover baking dish with foil and bake apples for 20 to 40 min. (see *Tip below) depending on how soft you want the apples. Remove foil and bake another 10 min. for the topping to caramelize slightly. Let cool for a few minutes before serving.
*Tip: Baking time is going to vary by apple size and type. We found McIntosh and Red Rome apples to soften the fastest, in about 30 min. total. Other varieties like Fuji, Red/Golden Delicious, Granny Smith, and Gala took up to 50 min. total to soften. Bake for less time if you want apples to hold their shape and still be firm enough to eat with a knife and fork. Bake longer if you want them to be spoon-soft.