Preheat the oven to 375°.
In a large bowl, mix together buttermilk, rolled oats, cranberries. and wheat bran. Let stand for 15 minutes.
Stir in honey, oil, sugar, and egg.
Whisk together flour, baking powder, baking soda, cinnamon, and salt.
Stir into oat mixture just until combined.
Stir in the apple.
Divide among 12 lined muffin cups.
Bake for 16-18 minutes, until muffin tops are firm to the touch.
Let cool completely.