Probably changed since, but from a long time ago (late 80's!), it was a sesh with the AFCO boss watching a few videos (including the cool 9 Sqn flying vid) followed by a discussion period where OASC, IOT and the reality of military life was mentioned to those who perhaps hadn't thought about it too much i.e. aircrew having to bomb stuff, blunties having to do ground defence.
As for the dress code, further comment shouldn't be required
I did see candidates actually turn up at OASC in jeans. Incredible.... Chop-tastic....
ST