Preheat your oven to 400°F (200°C).
Line a muffin tray with paper liners.
In a large bowl, mix the oat bran and buttermilk.
Let it sit for about 5-10 minutes to soften.
In another bowl, combine whole wheat flour, brown sugar, baking soda, baking powder, cinnamon, and salt.
To the oat bran mixture, add the egg and vegetable oil, stirring well.
Gradually add the dry ingredients to the wet mixture.
Stir until just combined.
Fold in the diced apples gently.
Fill each muffin cup about 3/4 full with the batter.
Bake for around 20-25 minutes or until a toothpick inserted into the center comes out clean.