I never heard this detail before. They tried to give him CPR? Which officers tried to do that?
There was a car waiting to take Oswald to his transfer destination, but instead of immediately putting him in the car and rushing him to the hospital, the DPD decided to carry him back to an office, call an ambulance, and wait for the ambulance to show up, which took 6-7 precious minutes.
As for the mind-boggling attempt at CPR, Detective Leavelle implied that an intern on duty from Parkland Hospital did it. Parkland would send an intern to the DPD on weekends as part of his/her training. Conveniently enough, Leavelle said the intern was the one who pressed up and down on Oswald's chest. If it was the intern, he must have been the most brainless intern ever born. The last thing in the world you do with a guy who's been shot in the gut is press up and down on his chest because, obviously, that's going to have the effect of pumping more blood out of any arteries that have been severed/punctured by the bullet. There is no indication that Oswald's heart had stopped or that he had stopped breathing, and, revealingly, the intern made no attempt to perform a trach on Oswald.
Ruby's bullet went from left to right, injuring the spleen, pancreas, aorta, vena cava, right kidney, and right lobe of the liver, and stopped in the right chest wall. Any intern with a lick of common sense would have suspected that a shot to the gut would cause such injuries and would have known not to press up and down on Oswald.