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 of orange juice and add it to the rind. Whisk in sugar, oil, and egg until mixture is smooth. Blend in oatmeal mixture, followed by the flour mixture. Stir until ingredients are combined, then gently fold in berries. Spoon batter into prepared muffin tins.
Bake for 15 minutes, or until a toothpick inserted into the center of the muffin comes out clean.