I'm glad you found that banana observation and the book referral helpful, teenvegan. You can try cutting out the bananas and see. Eliminate the bananas first and then the oats at a separate time so you can tell the difference between them. I use gluten free oats and have no problem with those. Oats do not contain gluten, but they can be contaminated with gluten in the harvesting and packaging process, unless they are processed and packaged separately from grains that contain gluten. More expensive, but that way I can have my oatmeal!
The way oatmeal is usually cooked, which is boiled, is not so much the problem, unless you end up having a problem with oats. To clarify, what I meant about baked, toasted grains is, for instance, the oats ground up for muffins, waffles, or pancakes, or toasted in the oven and browned-- any grain made into bread, crackers, chips, etc., cooked this way may be aggravating in insidious ways if it is browned too much. You might be helped by restricting grains cooked in that style. Sometimes with things a little is okay, sometimes not. You have to find your own tipping point, as well as which grains you can tolerate, and which ones you can't, no matter how they are cooked. It's a spectrum of experimentation. Hope that makes sense.
Happy Food Detective Work to you!