Oat bran and oatmeal contain comparable amounts of saturated fat, sodium, carbohydrates, protein and sugars, and nutrients such as calcium and iron. Oat bran contains about 50% more fiber and soluble fiber than oatmeal, providing 58% of an adult's daily requirement in a 1-cup serving. This makes oat bran more effective at lowering cholesterol and helping digestion. Oat bran also provides significantly larger amounts of nutrients such as potassium, phosphorus, folate and selenium as well as omega-3 and omega-6 fatty acids.
The main structural difference is that oatmeal (typically rolled oats) is whole grain, while oat bran is just the bran of the oat.
Because oat bran is creamier (due to the finer texture of the product), it satiates hunger more than oatmeal.