Something else that will help out in the upper-body development is to find a high pull up bar or something similar. You can practice the method of jumping up to grab it, pull yourself up, throw the chicken wing over(loosen the grip with say your right hand a bit, and roll it forward to throw your right elbow over the bar and hang on your arm pit instead of killing your biceps), wrap your left leg over the bar, and lift up and then kick down your (right) leg down to flip your body over the bar. You will spin right over it and then end up hanging from it. I also reccommend learning how to stop on top of it so that you are laying on your stomach on top of the bar.
You will use this technique for several different things at OCS. For example, there are a few high bars that you need to pull your body over on the O-course. You will also use this method if you are performing the commando crawl and fall off of the rope. It is also a good excercise to build upper body stregnth. I am not exactly the strongest when it comes to upper body, but when it comes down to the throw-your-body-over-some-high-shit, I can hold my own.
I built up a lot of this kind of stregnth and it really helped to get me through some of that upper body stuff that would have killed me otherwise. The thing that people go "oh $hit" about when at OCS and thinking about how unprepared they were is that they never focused on training while fatigued. All the big muscle guys think that they can hand over hand up the rope in 6 seconds because they've done it a thousand times. They dont take into account how exhausted they will be after putting out 100% on the O-course. The only way to actually get up there is using the technique. Something that I did was do 15 or so pullups (flexed arm hangs), sprint a short stretch, sprint back, do another 15 or so pullups and then try to throw myself over the bar.