Lowest possible standard for a Marine, which is way way above your average Cub Scout competancy, you understand.
Oswald did manage to shoot in the middle of the pack at Sharpshooter capability. He was competent enough.
We're only talking about 150 to 277 feet where the Dallas shots were made. That's not even 100 yards at the greatest range, and the first shot was at 50 measly paces. He had a sighted-in 4x scope on his Carcano shooting down from a kneeling prone position resting on a window ledge at a non-deflecting target. A Marine with even the lowly pizza-box (Marksman) shooting badge should be able to get at least one shot through a donut hole at that range under those circumstances. A part of Oswald's rifle qualification as a Marine, part of his score involved prone shooting at man-sized targets 500 (five hundred) yards away.
Oswald's rifle was capable of printing three round groups the size of a dime at 100 yards, according to FBI testimony using data they got from several shooters who were all NRA Master riflemen. They just noted that the strikes were off by several inches up and to the right. That's to be expected from a rifle with a mounted scope that's been dropped, mishandled, collected as evidence, fiddled with, and who knows what else before it went into evidenciary lockup. They knocked the scope off it's zero at some point between the time Oswald would have shot Kennedy and the time that the FBI ballistics lab reproduced the scenario a few years later for the Warren Commission. That's reasonable. They ran the tests using the rifle as it was collected without adjustment. If they even removed the receiver from it's stock, or the scope from it's mount, that's a complete write-off of the scope zeroing even if they otherwise treated it as a piece of fine china.
The Kennnedy Assassination was two 50 and 90 yard shots in 8 seconds, and conspiracy theorists have been making it out to be that Lee Harvey Oswald needed to be Annie Oakley to make those shots.