You stated he was a Marine sniper. He wasn't.
I have had training in homicide investigations and techniques, but I am not a homicide detective. I have had covert surveillance training and techniques, but I do not work for the CIA. I have taken multiple defensive driving courses, but I am not a test driver. I had some training regarding emergency medical techniques, but by no means am I a paramedic or a doctor.
Just because someone has had some type of basic training, does not somehow magically make them a professional at whatever they had training in.
An accurate statement would have been, Oswald had basic training in the Marines, that included some basic sniper training instruction, however he was not a Marine sniper. He was a Marine radar operator.
All Marines are riflemen - even the radar operators.
Oswald was a good shot; maybe not up to the Marine level of "sniper", but a good shot nonetheless - if he wasn't, he wouldn't be a Marine.