Preheat oven to 400°F. Trim ½ inch off the tops of the apples and scoop out the cores, leaving the bottoms intact. Arrange on a large microwave-safe plate. Cover with plastic and microwave 3 min.
Meanwhile, in a medium bowl, combine the flour, ¼ cup brown sugar, oats, and salt. Cut 4 tbsp butter into small pieces and work into flour mixture with hands until coarse crumbs form. Set aside.
Transfer apples to a large baking dish. Add ½ cup water to dish. In a small bowl, combine the remaining ½ cup sugar and apple pie spice and divide among apple cavities. Cover baking dish tightly with foil and bake 10 min.
Remove foil and fill each apple with streusel topping, pushing into cavities. Top with remaining 2 tbsp butter. Bake another 15 min., until apples are tender and streusel is golden brown.