Hi, that's really bad luck picking up an injury just when you want to be on top form for OASC. If you feel it's going to slow you down or prevent you from doing the bleep test, then yes you should advise your AFCO , who can decide a timescale for slotting you into the selection process.
In terms of advice for OASC - as Drewski says, there is a whole library of open source material available on this site and elsewhere, detailing virtually every second of your visit to Cranwell. You're going to have to find it and research it yourself, cos one of the big traits the selectors will be looking for is initiative. Nothing gets handed to you on a plate in the military, and if you really want the job, you'll have to work hard for it.
I managed to get selected for pilot in 1976, and went to OASC Biggin Hill armed with nothing more than 5 O levels (GCSEs), a big pot of enthusiasm, a slim brochure detailing the 3 day selection procedure, a head full of current affairs and some serious RAF knowledge gleaned from membership of the CCF and 10 years of digesting every single copy of Air Pictorial and Flight International I could get my hands on. No internet, no discussions with other candidates, I went in completely cold. That's the way it was in those days, and although the aptitude testing has become computerised, the person they want today is exactly the same as the person they wanted in 1976.
A career in the RAF is worth fighting hard for, so the question is, how badly do you want it?
Good luck mate, and let us know how you get on.