Preheat oven to 350ºF, line a square baking dish with aluminum foil, and give it a spritz with non-stick cooking spray.
Using a melon baller or a small spoon, remove the cores of the apples by scooping a well into middle about 3/4 of the way down. Sprinkle half of the cinnamon among the apples and set aside.
In a bowl, combine the oats, brown sugar, remaining cinnamon, melted butter, and salt and mix well. Nestle the apples into the prepared baking pan and stuff about 1/4 cup of the mixture into each of the apple wells.
Bake the apples for 30 to 40 minutes or until the crumble mixture is browned and the apples have softened.