I first had the On-Body Injector placed on the back of my arm. This placement was a problem when sleeping on my back or the same side that the On-Body Injector was on because it was in the way. It felt uncomfortable and I worried that it might disconnect from my arm. I also had problems removing it with it on the back of my arm.
After having it on my arm twice I switched to having it on my stomach. Now I can sleep on my back and left or right hand side. Also, I seem to have less pain. I think that might be because there is little fat on the back of my arm however there is some fat on my stomach. I also have both hands free to remove it when it is done injecting.