Why oatmeal works for bulking
Bulking requires a consistent calorie surplus. Oatmeal helps because it is cheap, repeatable, easy to digest for many people, and simple to customize from a modest meal to a high-calorie bowl.
The base oats provide carbs and fiber. Milk, whey, peanut butter, banana, honey, nuts, seeds, and dried fruit can raise calories quickly when appetite is the limiting factor.