This is all about hydration and gelatinization of starch. Similar principles apply to corn and oatmeal, but I'll concentrate on the latter since I'm very familiar with it from making porridge.
Peak gelatinization for the starch in oatmeal occurs at about 95C. A gentle simmer is fine, but a racing boil will cause granules of unhydrated starch to form. (Something similar happens if roux is added to a too hot sauce.) Likewise, gentle stirring helps, but vigorous whisking will break up the suspension. Also I suggest you deliberately allow a thin layer of oatmeal or corn to stick to the bottom of the pan: it seems to have extra thickening powers once it is scrapped off and incorporated thoroughly into the mixture. Towards the end, you can deliberately let a skin form on the top before stirring it back it.
Finally it is critical to add water slowly: whenever you feel the suspension thicken, it's time to add a drop more.
I've found that milk can actually make a less creamy porridge, perhaps because the fat gets in the way of the starch matrix. So I'd suggest preparing the oatmeal with water, and then adding milk/cream only at the end.
Another huge factor is how much your oatmeal or corn has been preprocessed. In the UK one can buy 'instant oats', which have already been finely rolled and partially cooked. These hydrate very quickly. At the other extreme, pinhead oats really have to be soaked overnight. I have had good results by going further and partially cooking it, to the point where the starch granules have visibly broken up. Even treated in this way, it can take 20 minutes or more to form a creamy porridge the next morning.
