Preheat oven to 350. With mixer or pastry blender, beat cake mix, 5 tablespoons of the butter, and 1 cup of the oats. Mix until crumbly and then set aside 1 cup of this mixture for the topping.
Combine egg and the remaining mixture and stir well. Press into 9 x 13" pan that has been greased and floured. Spread the pineapple pie filling evenly over the crust.
In the same bowl, combine the reserved crumb mixture, the remaining oats, 3 tablespoons butter and the brown sugar and blend well. Sprinkle over the pineapple filling and bake for about 35 minutes or until light brown on top.