Crust and streusel:
2 C oats, uncooked
2-1/2 C flour
1-1/4 C sugar
2 t baking powder
1 C (2 sticks) butter, melted
3 C peeled, thinly sliced apples (about 3 medium apples)
2 T flour
12-oz jar (1 C) raspberry or apricot preserves
Heat oven to 375 F. Combine oats, 2-1/2 C flour, sugar, and baking powder, mix well. Add butter, mixing until moistened. Reserve 2 cups, set aside. Press remainder in bottom of 13×9 inch baking pan. Bake 15 minutes.
Combine apples and 2 T flour, stir in preserves. Spread on crust to within 1/2 inch of edge. Sprinkle with remaining oat mixture, pressing lightly. Bake 30-35 minutes until light golden brown. Cool, then cut into bars.