Let's say a pot of raw dry oats becomes fully cooked in 5 minutes. Just oats and water. How is it considered chometz?
The Shulchan Aruch (OC 555:3) rules that if one kneaded his dough with hot water it is forbidden, the assumption being that heat makes something become chametz quicker. This is also seen in the rule (559:1) not to knead the dough in the sun or in a room with a hot oven. Moreover one can leave dough in cold water indefinitely to prevent its becoming chametz (557:2).
Similarly, all oats are heat-treated to prevent them from becoming rancid; if this heating is done with “wet” steam, the oats/oatmeal may be chametz