The fibers contained in oat bran have two important properties.

First, it is very absorbent and therefore fill the stomach quickly and cause satiety.

Secondly, it is extremely sticky. Thus, when they reach the small intestine, "entrap" other foods surrounding the oat bran, preventing their passage into the blood.

Essentially, that lead to loss of calories in the intestine that reaches 30% of the calorie intake.