Heat oven to 350°F. Grease 13x9-inch pan. In medium saucepan, combine all filling ingredients. Cook over medium heat until slightly thickened, stirring constantly. Cool slightly.
Meanwhile, lightly spoon flour into measuring cup, level off. In medium bowl, combine flour, oats, 3/4 cup brown sugar and baking soda; mix well. With pastry blender or fork, cut in margarine until mixture is crumbly. Reserve 1 1/2 cups oat mixture for topping. Press remaining oat mixture evenly in bottom of greased pan to form base.
Bake at 350°F. for 10 minutes. Remove pan from oven. Spoon filling evenly over partially baked base. Sprinkle with reserved topping. Return to oven; bake an additional 20 to 30 minutes or until light golden brown and set. Cool 1 hour or until completely cooled. Cut into bars.