Chicks are better off with just their starter, it's well formulated to make them grow. The oatmeal cuts the nutrition in the starter, could be a source of pasty butt (because it increases the starch in the diet) and....it is a rather bad idea to give them while in the developing stage because it contains the anti-nutritional factor beta glucan. Anti-nutritional factor means something that adversely affects absorption of nutrients. I highly recommend stopping the oatmeal immediately and then very careful observation to see if the problem goes away, hopefully things are not too bad, since the chick is behaving normally. But if the problem won't go away quickly after eliminating the oatmeal, medication will be needed and it will be a guessing game to pick something (probably an antibiotic first imo but we aren't there yet).
Beta glucan generally speaking though, it's something good for the immune system, it's anti-cancerigen, it removes toxins from the body, etc. In humans. But birds are not much different in their immune system response. What I think is that it's wrong to give oatmeal to chicks until they grow up and it's wrong to present adult chickens with a big bowl of oatmeal, but... just a very little bit of oatmeal not often would be beneficial (again just for adult chickens).
