Man, you have been taken. If you haven't paid the guy yet, don't. He needs to tear the whole thing out and build is as you specified.
You are correct about the OSB, it will always show a pattern through and to top that off, yellow pine is notorious for peeling paint; it is almost impossible to keep painted.
Yes, you could buy a sheet of thin oak plywood to cover the risers but, then the treads would look awful. Its a tear out and replace.