Yes oatmeal can make you constipated if you eat too much oatmeal.
Oatmeal containers lots of fiber and in moderation the oatmeal is good for you and can help you poop and may relieve constipation.
But overeating on oatmeal or any other high fiber foods can lead to constipation.
Too much of that fiber can cause your poop to harden and make it hard to pass the poop through your intestines.
So that's why it becomes hard to poop sometimes and you sometimes become constipated as a result of eating too much oatmeal.