Oswald immediately lost consciousness. That does not happen to all abdominal gunshot victims. So, he lost consciousness, and he lost it immediately. Why would he lose consciousness? There can only be one reason: lack of blood to his brain. His circulation must have collapsed (assuming he was really shot, that it wasn't a hoax). His circulation collapsed because his major blood vessels were cut. So, how long can a person live with no blood going to their brain?
That ambulance was just a glorified taxi. Oswald received no medical care in it, and therefore, there was no reason to wait for it. He should have been rushed to the hospital immediately in whatever vehicle was right at hand.
So, why wasn't he?
Some may say it's because they wanted him to bleed to death, but the thing is: he would have. I really think he would have. So, I think it's more likely that he wasn't shot until later. Exactly when I don't know. I am still studying it.