I have oatmeal most mornings -with raisins and almonds. It has never been a problem for me. If you have to go to the bathroom while eating it, it isn't the oatmeal. It is your gastro-colic reflex kicking in. When I am flaring, whatever I eat first thing in the morning, even a drink of water, will cause me to go to the bathroom. After all, what you are eating hasn't reached your colon yet.
It's so weird you posted this because I was just coming back on here to say that come to think of it, no matter what I eat in the morning, I usually have to stop mid-way through to go to the bathroom, so most days I don't eat breakfast because it increase my bathroom trips by quite a bit. Maybe I should try eating oatmeal at night and see how I do.
What is the gastric-colic reflex? Is it related to having UC? I had colic when I was a baby, I always wonder if any of this is related, but they always say it's not.