Preheat oven to 325 degrees F. Line a 15x10x1-inch baking pan with foil, extending the foil over the edges of the pan. Lightly coat the foil with cooking spray; set aside.
In a large bowl stir together oats, 1/2 cup of the pistachio nuts, the honey, flour, orange peel, orange juice, brown sugar, butter, vanilla, and baking soda; mix well. Press mixture into prepared pan.
Bake about 20 minutes or until set in center and lightly browned. Cool completely in pan on a wire rack.
In a small microwave-safe bowl heat chocolate pieces on 50 percent power (medium) for 2 to 2 1/2 minutes or until melted and smooth, stirring twice. Spread melted chocolate over cooled bars in pan. Sprinkle with the remaining 1/4 cup pistachio nuts. Let stand until set. Using the edges of the foil, lift uncut bars out of pan. Use a sharp knife to cut into 70 (about 2x1-inch) bars.