Spray at 9 x 13-inch baking dish with non-stick spray.
Peel apples and slice into wedges and sprinkle with cinnamon.
In sauce pan combine cornstarch, cold water and sugar. Heat on medium high heat to a boil stirring continually. Mixture will become a thick syrup. Remove from heat.
Mix flour, brown sugar, oatmeal and butter together by hand. Press half of mixture into the bottom of the prepared pan. Place apples evenly on top. Pour syrup over apples. Then top with remaining oatmeal mixture to make crumbs.