Preheat oven to 375 degrees. In a small bowl, toss blueberries with 2 teaspoons of granulated sugar; set aside. In another small bowl, toss strawberries with remaining 2 teaspoons of granulated sugar; set aside. In a third small bowl, stir together oats, brown sugar and butter. Spread mixture in a single layer on a sheet pan; bake, stirring occasionally, until lightly browned, about 15 minutes. Spread blueberries and strawberries in an 8 x 8-inch glass dish, alternating to create stripes. Sprinkle with oat topping, decorate with whipped topping and serve.