Meanwhile, whisk together rolled oats, flour, granulated sugar, brown sugar, cinnamon and salt; with fingers or pastry blender, blend in butter until crumbly. Press two-thirds evenly into 9-inch (2.5 L) square cake pan. Bake in 350°F (180°C) oven until golden brown, about 20 minutes. Let cool in pan on rack for 5 minutes.