Preheat oven to 350°.
Beat sugar and margarine at medium speed of a mixer until light and fluffy. Add orange flavoring and egg whites; beat well. Add yogurt; beat well. Lightly spoon flour into dry measuring cups, and level with a knife. Combine flour, oat bran, orange rind, baking soda, and salt in a small bowl. Gradually add flour mixture to sugar mixture, stirring just until moist.
Coat an 8-inch loaf pan with cooking spray; spoon cake batter into pan. Bake at 350° for 1 hour and 5 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 10 minutes on a wire rack; remove from pan. Cool completely.