Preheat oven to 375°F (190°C). Grease a muffin tin or line with paper liners.
In a large bowl, combine oat bran, grated carrots, and raisins.
In another bowl, whisk together flour, brown sugar, baking soda, cinnamon, and salt.
In a third bowl, mix buttermilk, egg, vegetable oil, and vanilla extract.
Combine wet ingredients with dry ingredients and stir until just combined.
Fold in the oat bran mixture.
Spoon batter into prepared muffin tin, filling each cup about ⅔ full.
Bake for 20-25 minutes, or until a toothpick inserted into the center comes out clean.
Let muffins cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.