Josh
I am not sure that anyone but a current Boarding Officer at OASC could give you an accurate answer to your main question. How many PC versus SSC places OASC offer is a very delicately balanced equation and depends on the future plans for the branch and the career options that people within the branch are making. I dare say that the OASC targets change monthly, if not weekly.
I don't think there is any doubt that asking for a PC shows greater commitment to the RAF, but it probably gains you something like a .5% bonus, and I bet there are many other ways you could earn that little extra in the OASC process, so it is not massively significant.
I also think there is some logic to the suggestion that OASC would be more likely to offer a PC to the strongest candidates. However, that does not mean that you are a weak candidate if you get or go for a SSC. The RAF offers SSCs for a reason, because it wants them and it knows that some applicants want them, so it's not a sign of a lack of commitment to ask for a SSC.
If you really want to go for a PC for career reasons, rather than just to try to improve your selection chances, then ask when at OASC.
As to whether acceptance for OASC indicates a place at IOT, I'm afraid not. Depending on the severity of the filter interview that you have gone through, and you will note that people on TSR are having very different experiences at filter, you will be better or less well prepared for OASC. But many many more people get to OASC than get to IOT. I'm afraid that I don't have the exact figures to hand, but I am sure they are on TSR somewhere. Suffice to say that getting to OASC is only one step in the process, being selected and offered a date for IOT is a much bigger achievement.