Preheat oven to 350 degrees. Combine blueberries, brown sugar, lemon juice and cinnamon and place in a buttered 6 cup baking dish. Place flour in mixing bowl and add butter and rub or cut in until mixture resembles breadcrumbs. Add sugar, cinnamon and rolled oats, and mix well. Spread the mixture over the berries and pat firm. Bake at 350 until berries are softened and bubbling and topping is brown. Serve warm with cream, custard or ice-cream. Makes 6-8 servings.