No actual figures for OASC pass rates but:
"the high pass rate (esp for fg trg) is due to the exceptional job the occ pyschs do in making the aptitude tests robust and relevant. The Gp Capt (the post is now Wg cdr) used to spend 80% of his time with the psychs, esp . . . introduced ab initio for typhoon and . . . making plans for ab initio UAV Pilot selection"
Also:
"The medical process is also hugely robust and instantly weeds out any doubt and in my opinion is the best of the 3 services."
So basically, while suitable to go to OASC, OASC assessment of pilot/aircrew potential is very robust hence the high success rates into IOT and beyond.
My figure of 40-45 will not be far off the mark considering the capability of the two FJ OCU to accept ab initio students with numbers of previous FJ aircrew re-roling from the other types (F3, Harrier, Jaguar).