This is the position of Oswald's arm when he was being removed from the ambulance.
So, his arm was externally rotated, raised to 90 degrees, and his elbow also bent to 90 degrees.
Considering the state he was supposedly in, he could not have done that himself. Here is where his right arm was when they put up in the ambulance.
So, how did it go from that to this?
DON'T tell me that Oswald did it. Oswald was non-responsive in the jail office. The Bieb thought he was dead. He couldn't detect a heartbeat or respiration. His eyes were partially dilated. And with each passing second, he could only be getting more incapacitated and that much closer to death. So, how could he lift and swing his arm up through the widest range of motion possible? He couldn't. He wouldn't. He didn't. Somebody else did that.