Preheat the oven to 350 degrees.
Add the chopped apples to a medium-sized bowl. Set aside.
In a bowl, mix together the flour, oats, brown sugar, cinnamon, and salt and stir to combine. Add in the butter and rub it into the flour mixture until the mixture resembles coarse bread crumbs. Note: Keep the butter cold; if you notice it's getting warm, pop it in the fridge for a few minutes until it's chilled again.
Add 1/4 cup of the flour mixture to the bowl with the apples and toss to coat.
Divide the apples into 4 ramekins (if you don't have ramekins, you can add it all to a small baking dish).
Divide the flour and butter topping evenly over the top of the apples.
Place the ramekins on a baking sheet and bake for 25-30 minutes until the top is golden brown and the apples are slightly bubbly.
Allow to cool for a few minutes and serve warm with vanilla ice cream or whipped cream.