Firstly, unless you've already been to OASC - which judging by your post it doesn't look like you have - there's no way you're going to make either of the next two IOTs. If you started your application process this week then if you're lucky you might get on the first course of 2010, but I wouldn't hold your breath, it's extremely busy.
A previous undertaking of the aptitude tests is no bar to an application, but it could have an effect on how soon you'd be able to retake them. Chances are, though, that the GAPAN tests are sufficiently different to the RAF's that you'll be able to do it again whenever.
In terms of content, I can't help you much, except to say that yes, you are required to do calculations as part of your OASC aptitude tests.