@turquoise, @ddljohn-- I use to give my kids oatmeal baths when they had diaper rash as infants and toddlers. It used to work great! It also saved my daughter from unbearable pain from sores when she caught chickenpox in primary school. She had sores all over her body and the oatmeal bath made her feel much better.
I agree that the thinner the oatmeal, the better the results are but I've never heard of colloidal oatmeal before. I just grind regular oatmeal until it's pretty thin and add it to my bath water. Collodial oatmeal bath is probably a way to get people to pay more for good old oatmeal.
I think the most important part is putting the oatmeal in a sack so that it doesn't clog the tub. Those resealable tea bags were excellent for that.