To make an 8" x 8" pan, increase ingredients to the following:
4 apples, 1/2 cup sugar, 1 tsp cinnamon, 1 cup oats, 3/4 cup flour, 1 cup brown sugar, 1 tsp cinnamon, 1/2 tsp salt, 1/2 cup butter. Baking time increases to 45-50 minutes.
You can replace apples with blueberries. For 8" x 8" pan, you'll need 5 cups. Reserve about 1/4 cup to sprinkle over the streusel before baking.