Adjust oven rack to top third position; preheat oven to 400 degrees F. Coat a 12-cup muffin pan with vegetable spray, including top edges as muffins rise above pan.
Using a whisk, thoroughly combine flour, sugar, baking powder, baking soda, and salt in a large bowl.
Puree orange in food processor and transfer to a medium bowl. Whisk in egg, milk, and oil until blended, then pour over dry ingredients. Fold in with rubber spatula just until combined; do not overmix.
Spoon batter into prepared muffin cups, dividing it evenly. Bake 20 minutes or until golden. Turn out onto rack and serve warm.