In my filter interview we had a fairly informal chat after the main part, in which he gave me general advice if i was called up for OASC.
If you have done well at your filter, your interviewer will recommend that you attend OASC. The decision of whether you will actually attend rests with OASC, and the speed with which you may be given a date can vary depending on the branch's you have applied for.
You should recieve a letter in a couple weeks saying that OASC has recieved your file, and then after more waiting you will recieve another letter with an OASC date.
From my filter to my OASC date it was exactly 4 months.