Heat oil in a large nonstick skillet over medium-high heat. Add onion to pan; sauté 3 minutes or until onion is tender. Set aside.
Weigh or lightly spoon flour into dry measuring cups; level with a knife. Combine flour, baking soda, and salt in a large bowl; make a well in center of mixture. Combine buttermilk, butter, and egg whites, stirring with a whisk. Add buttermilk mixture to flour mixture, stirring just until moist. Fold in onion, olives, and oregano.
Spread batter into an 8 x 4–inch loaf pan coated with cooking spray. Bake at 350° for 45 minutes or until a wooden pick inserted in center comes out clean. Cool 10 minutes in pan on a wire rack; remove from pan. Cool completely on wire rack.