Preheat your oven to 350F and line a baking sheet with parchment.
Core your apples with an apple corer, spoon, or melon scoop (see instructions above).
Place the flour, sugar, oats, cinnamon, and salt into a mixing bowl. Drizzle in the melted butter. Use your hands to incorporate everything until you have a crumbly, streusel-style mixture.
Spoon the filling into the cored apples. Try to pack them a little, but don't press or fill them with too much force. Spoon any leftover streusel onto the baking sheet to bake with the apples (sort of like granola).
Bake the apples for 30-40 minutes (this will depend on the size of your apples), or until the top of the filling is browning and the apples are easily pierced with a paring knife.
Allow the apples to cool for at least 10 minutes before slicing into them and enjoying.