Oswald actually received a sharpshooter grade in his early days in the Marines, then later achieved a lower "marksman" score. He was overall considered an excellent shot vs the average American, and slightly above average in the Marines.
While true Oswald was a Marine marksman, it's also true that that is the lowest qualification a Marine can have. They are in order: expert, sharpshooter and marksman. His marksman rating is always used in a way to mislead the public because they don't know the difference. Make no mistake, he was a lousy shot at best.