The working of the digestive system is based on two things:
1. Muscular Action – The gut is a muscular tube and its squeezing action is very important. It helps to break up the food into very small pieces with a large surface area for enzyme action to take place. It also helps the chemical breakdown of the food, mixing it with various digestive juices, which break down its molecules. The muscles also move the food along the gut from one area of chemical digestion to another. willy
2. Enzyme Action – enzymes are chemicals which speed up (catalyse) other reactions. They do not actually take part in the reaction or change it in any way except to make it happen faster. Each type of enzyme acts on a specific type of food.
Grinding oats would create more surface area from the start making it easier to digest