Preheat oven to 400 degrees.
Trim 1/2 inch off the top of the apples and remove cores.
Cover and microwave for 3 minutes.
In a mixing bowl, combine the flour, 1/4 cup brown sugar, oats and salt. Cut four tablespoons of the butter into small pieces and mix them into the flour mixture using a fork or two knives. The mixture should resemble large crumbs.
Transfer apples to a baking dish. Add 1/2 cup water to the bottom of the dish.
In a small bowl, combine remaining sugar and apple pie spice. Spoon this mixture into each apple.
Cover baking dish tightly and bake for 10 minutes.
Remove baking dish from oven and uncover. Fill each apple with the oatmeal topping from the mixing bowl. Cut remaining butter into 6 pieces and dot the top of each apple with the butter.
Bake 15 minutes, until apples are tender and streusel topping is browned.