Had 2 shots at OASC. Failed miserably the first time. The explanatory letter sent to my OC just wasn't written about me! However, I swallowed my pride and spent the next 2 years asking every officer I could lay my hands on just what they did and what their reaction was. The answers varied: "Wasn't particularly bothered, I had another job lined up" etc etc etc. Bottom line that I worked out was that they were being themselves when they went for selection. And that's what I did on the second occasion - and passed.
That reason became even clearer when I was posted as a flight commander at IOT and which I developed in cadets when I and a number of other flight commanders started the Recourse Squadron (now disbanded because the Recourse syllabus was eventually adopted for the Main Course).
BE YOURSELF!