Preheat oven to 400° F. Lightly coat 12 muffin cups with cooking spray. Stir together oats and buttermilk in a small bowl and set aside for 5 minutes.
Whisk flour, baking powder, baking soda, cinnamon and salt together in a medium bowl. Grate rind from orange and add to a large bowl; squeeze 1/2 cup orange juice and add to rind. Whisk in sugar, oil and egg until mixture is smooth. Blend in oatmeal mixture, followed by flour mixture. Stir until ingredients are just combined, then gently fold in berries.
Spoon batter into prepared muffin tins and bake for 15 minutes, or until a toothpick inserted in center of muffin comes out clean.
