First, are your corn and oats whole, cracked, rolled? Whole grains are not digestible, this is why you see them in the stool. The shell of the grain is not penetrable by enzymes or microbials, preventing them from getting to the germ which is the part that actually contains nutrients. Even animals with two sets of teeth have trouble cracking the shell on corn when chewing, its nearly impossible for ruminants that have only a lower set. So when making diets for cattle it is important to make sure your grains are cracked, rolled, flaked, etc. As long as they have had some sort of processing which breaks the outer shell.
Also if your ration consists of 50% corn and 50% oats then this is not a complete feed, and if a calf is not on complete feed then 2 flakes of hay a day may not be enough fiber in his diet to keep his stool firm. I would also be concerned about the protein intake of your calf, with just corn and oats you dont have a good protein source. Oats are pretty much just a filler in most peoples eyes. I am no nutritionists, genetics is more my thing, but I have had to take a few nutrition courses. I would consider replacing some of the oats with a good protein source such as soy bean meal and adding a fiber source such as cotton seed hulls. You also need to add some type of vitamin and mineral pack in there as well.