There actually have been clinical trials for managing IBS through diet, e.g. the low-FODMAP diet. Oatmeal doesn't sound like a very plausible cure for IBS, but

There actually have been clinical trials for managing IBS through diet, e.g. the low-FODMAP diet. Oatmeal doesn't sound like a very plausible cure for IBS, but whatever they used to eat for breakfast being a problem food for them seems very plausible.