In a large bowl, combine the flour, oats, 1 cup sugar and baking soda. Cut in butter until mixture resembles coarse crumbs. Reserve 2 cups for topping. Press remaining crumb mixture into a greased 13-in. x 9-in. baking pan; set aside.
In a large saucepan, combine blueberries, cornstarch, lemon juice and remaining sugar. Bring to a boil; boil for 2 minutes, stirring constantly.
Spread evenly over the crust. Sprinkle with the reserved crumb mixture. Bake at 375° for 25 minutes or until lightly browned. Cool on a wire rack. Cut into bars.