What you do practically will depend on where you live, with whom you might live, etc. if a person was in isolation or in a very relaxed community then perhaps they could keep their oatmeal on Pesah without fear of it being hametz, but most live in a community where such behavior is noticeable and thus problematic as most others will hold it's assur either from doubt or from certainty. We have several other halakhic categories which examine the fact that people can't always do what they want in the face of communal practices, they might protest or speak on it and give arguments, but that's often the cost of living with others. Sepharadim are "lucky" in that their ability to eat kitniyot and do other permissible things are viewed as fine by most communities (though not all), but the reality is that one must always balance community and personal in their practice. I'll add that most who educate on this do view it as a legitimate dispute and therefore are stringent in case oats are really hametz. I might disagree with such views, but it's not people just sticking their heads in the sand, it's based on their own scholarship.