My experience was probably somewhat different, given than I was doing it between my first and second year of dental school. They knew I didn't know much, so there wasn't much "pimping". Plus, I went in August, so they had just gotten their new interns and were more apt to pimp them.
Overall, I'd go in at ~6 AM (first in other than the interns) and leave when they told me to. Most days it was around 5 PM. I hung out with the third years mostly - the other years were on rotations a lot of the time. I saw as many OR cases as possible and also saw them work the clinics. The hospital I went to had OMS teams that alternated - one team was in the OR on Tues/Thus and the other on Wed/Fri. Whoever wasn't in the OR was in the clinic. I basically jumped back and forth between the teams. I asked to take call with one of the residents but he said that since the call schedule was for a hospital that was off-site, I wasn't allowed. I'm going to try to make sure that I see some call in my upcoming externships (I have three in the Spring).
All in all, it was a great experience. I'd highly recommend trying to grab the program director for a second to sit down and chat. I did and I think it was good. If you have specific questions, let me know.