Dragging the thread back to the subject
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Do you mean the interview before OASC or the interview at OASC?
The filter interview is really there to see if you're the kind of chap that OASC should have a look at, whereas the interview at OASC is there to see what kind of chap (or chappess) you are.
Swotting up a bit for the filter interview is the best advice I could give, not just on your favoured bit of machinery, Chinook maybe, but all the aircraft that NCA fly. The interview will be a one-on-one with an Officer, although it could be a Warrant Officer/Master Aircrew, and his/her recommendation will be from his/her perception of you during the interview.
The OASC interview (which everyone seems to be afraid of, but shouldn't be) will be in front of a board (3 I think?), the other tests will be marked or assessed by other selection staff so the whole result is a group decision.
Therefore, be smart as advised, have a good base knowledge of the job/aircraft/locations/operations (it doesn't need to be A-Level stuff), have 3 or 4 current affairs subjects up your sleeve as well. Most of all relax and try to give your best.
Good luck!
P.S. Downsizer, you'll have someone's eye out with that.