Hiya link4,
I have been assessed by OAT and am starting in Feb.
The assessment is not all that difficult, it is just like any other assessment that you would go through for a job, but this one lasts for 2days. The psycho and apt tests are not that bad. By their nature you cant really "revise" for them but do brush up on your maths and physics. Just simple stuff like long division and multiplication. It really is GCSE stuff so if you have got a C or above, with a bit of brushing up, you should be fine.
Make friends with the people that you are on the course with too. We all went out for a meal in Oxford on the 1st night and that helped us all in the team building exercises the following day. And most of all, just relax during your interview.
Be ready for a bombardment of "you shouldnt go to OAT" on these pages too.
It is expensive, but if you have got the money and the love of flying then go for it!
Good luck!!