PREHEAT oven to 375° F. Grease 9-inch-square baking pan.
COMBINE flour, oats, granulated sugar, brown sugar, baking powder and salt in large bowl. Cut in butter with pastry blender or 2 knives; reserve 1 cup oat mixture for topping. Press remaining oat mixture into prepared baking pan. Spread preserves over crust; sprinkle with morsels.
COMBINE reserved oat mixture with nuts; sprinkle over morsels. Pat down lightly.
BAKE for 30 to 35 minutes or until golden brown. Cool completely in pan on wire rack until chocolate is firm or refrigerate for 30 minutes to speed cooling. Sprinkle with powdered sugar.