Only counting people who actually took the test (since several left because they were getting fed up waiting) first time pass rate while I was there was 60%, although that was only a small sample (five people) over two months.
I think the main examiner that OBA uses is a tough examiner, but don't read that as an unfair examiner. There are figures floating around on here (as well as in the OBA restaurant) that suggest he has a 40% pass rate, but I am not sure if this is correct or not. It shouldn't make a difference anyway - if you are up to scratch you are up to scratch, it's just what level of scratchness the examiner thinks is worthy of a pass that might vary slightly! Basically, your flight test will probably be an... emotional experience with either outcome!