Preheat oven to 375β.
Peel, core, and slice apples into thin wedges. Spread apples and blueberries in the baking dish.
In a mixing bowl, stir together the oats, brown sugar, flour, and cinnamon.
Add the softened butter in chunks and rub it into the oat mixture with your fingertips (or use a pastry cutter) until well incorporated and crumbly.
Crumble the topping evenly over the fruit and loosely cover with foil.
Bake, covered, for 20 minutes. Then remove the foil and bake uncovered for an additional 25β30 minutes, until the topping is golden brown and the juices are bubbling.
Let sit for 5 minutes before serving.