Oswald had the motive, desire, obsession.
Physically, Oswald had the correct angle & time required. Oswald had over 8 seconds to fire 3 shots. Senior Secret Service agent, (wheel man/ limo driver) Bill Greer nearly stops the vehicle as he turns towards JFK following the 2nd shot. In so doing he unknowingly provides Lee with the opportunity for the fatal head shot. It is this simple. One man, obsessed, kills the president, his opportunity made possible by the open limo, direction & more importantly the speed of the limo.