What feedback did you receive as to why you failed the interview?
Be positive, project yourself and ennunciate, don't bull$hit; if you don't know the answer to something then say so.
It's pretty fast paced, so try and be concise. If one of the boarding officers cuts you off and proceeds onto another topic or area then that was all they wanted to hear and are moving on! Do a dry run in your head beforehand; think about your replies to areas such as participation in team sports, school and university clubs, cadets/UAS, previous demonstration of responsibility and leadership etc. Why do you want to be a pilot? Why do you want to be an officer? Which is more important? What if you fail pilot selection but are offered a ground branch? If you don't want to be a Nav, how do you explain it to the Navigator Sqn Ldr across the desk who is interviewing you? What do you think about recreational drugs?
There are loads more similar to the above. Current affairs and Service knowledge forms a fair chunk of the interview. If you haven't been in the ATC/CCF, or taken part in team sports, then have a reason why. The majority of candidates failing interview at OASC do so because they do not come across as being positive enough.
Good Luck. And try and relax when you're there.
ST