That fact is well established and is reported on at some length in the WCR. He hit a target at 200 yds to 98% accuracy.
In accordance with standard Marine procedures, Oswald received extensive training in marksmanship. During the first week of an intensive 8-week training period he received instruction in sighting, aiming, and manipulation of the trigger. He went through a series of exercises called dry firing where he assumed all positions which would later be used in the qualification course. After familiarization with live ammunition in the .22 rifle and .22 pistol, Oswald, like all Marine recruits, received training on the rifle range at distances up to 500 yards, firing 50 rounds each day for five days.
Following that training, Oswald was tested in December of 1956, and obtained a score of 212, which was 2 points above the minimum for qualifications as a "sharpshooter" in a scale of marksman--sharpshooter--expert. In May of 1959, on another range, Oswald scored 191, which was 1 point over the minimum for ranking as a "marksman." The Marine Corps records maintained on Oswald further show that he had fired and was familiar with the Browning Automatic rifle, .45 caliber pistol, and 12-gage riot gun. - Source, WCR, pg 191
The kill shot to JFK's head was from a distance of 88 yds, well within LHO's abilities. The USMC testified to the difficulty of the shots made by Oswald:
Four marksmanship experts testified before the Commission. Maj. Eugene D. Anderson, assistant head of the Marksmanship Branch of the U.S. Marine Corps, testified that the shots which struck the President in the neck and in the head were "not ... particularly difficult." Robert A. Frazier, FBI expert in firearms identification and training, said: "From my own experience in shooting over the years, when you shoot at 175 feet or 260 feet, which is less than 100 yards, with a telescopic sight, you should not have any difficulty in hitting your target. I mean it requires no training at all to shoot a weapon with a telescopic sight once you know that you must put the crosshairs on the target and that is all that is necessary." - Source, WCR, pg 190
Representatives from the USMC testified to the WC that Oswald was a "better than average shot.":
Major Anderson concluded:
"I would say that as compared to other Marines receiving the same type of training, that Oswald was a good shot, somewhat better than or equal to--better than the average let us say. As compared to a civilian who had not received this intensive training, he would be considered as a good to excellent shot."
When Sergeant Zahm was asked whether Oswald's Marine Corps training would have made it easier to operate a rifle with a four-power scope, he replied:
"Based on that training, his basic knowledge in sight manipulation and trigger squeeze and what not, I would say that he would be capable of sighting that rifle in well, firing it, with 10 rounds."
After reviewing Oswald's marksmanship scores, Sergeant Zahm concluded: "I would say in the Marine Corps he is a good shot, slightly above average, and as compared to the average male of his age throughout the civilian, throughout the United States, that he is an excellent shot." - Source, WCR, pg 192
Oswald scored as a Marksman on the shooting he test he took right before he left the USMC. Why conspiracy theorists cite this lower score and ignore his ability as a sharpshooter escape logic.
More important, Oswald practiced his shooting after he left the USMC. He joined a shooting club in Minsk and went hunting with his brother and others when he returned to the USA. Marina testified that he also took shooting practice.
As far as "rehearsing" the assassination, LHO did just that - look up the tale of LHO's attempt on the life of Gen. Walker.
The Zapruder film shows that LHO had 8.5 seconds to make his three shots. The clock starts running when he takes that first shot as the first round was already chambered. That means that LHO had almost the full 8.5 seconds available to him to chamber and fire the final two shots. Shooters hired by the WC to duplicate LHO's feat were able to fire the 3 shots WITH ACCURACY in as little as 4.4 seconds.
So, you're "40 years old and have been fascinated by the case since I was 10." Yet in that 30 years, you haven't been fascinated enough to read the WCR and to avoid making yourself look stupid by saying that Oswald was a poor shot and wasn't a sharpshooter. Maybe you don't want to know that LHO scored higher than did you.