Bring brandy to a boil in a small saucepan and remove from heat. Stir in prunes and let stand while making topping.
Pulse together flour, 6 tablespoons brown sugar, butter, salt, and cinnamon in a food processor until mixture resembles coarse meal. Add oats and pulse once or twice to just combine.
Peel, quarter, and core apples and cut into 1/2-inch wedges. Transfer apples to a 1 1/2-quart shallow baking dish and toss with prune mixture and remaining 2 tablespoons brown sugar.
Sprinkle topping evenly over apple filling and bake in middle of oven until topping is golden brown and filling is bubbling, about 30 minutes.
