Oswald was a marine sharpshooter, and it took him 3 shots with a scoped rifle (probably at point blank range, though I don't know the ballistics of that gun). That suggests it wasn't Oswald. Though even then Oswald clearly would have been standing next to the unknown shooter urging him to do it.
For the record, a "marine sharpshooter" is an almost meaningless term. Virtually any physically healthy person is capable of meeting this bar with an hour or two of practice. It sounds way more impressive than it is. Oswald was in no way a highly skilled shooter. In fact, he had extremely basic firearms skills.
