Just google pilot aptitude tests etc and you will get a plethora of links of things to do.
When you apply for assessment the school will give you a list of things they will test you on, theres no magic formula so just get revising. Having been to oaa recently I'd say there's nothing in particular you can revise for in the group exercise. I dont think I wowwed anyone on my interview but was just honest about my aspirations. If you do anything intersting besides getting up for work, try mention that too!