If you are set on OAA then it may be useful to look for a club around that area (White Waltham, Wycombe Air Park, Enstone, Turweston to name but a few) to get used to the area that you will be learning/testing in.
I have just started a CPL at Stapleford and there isn't an awful lot of difference to the PPL syllabus. The main emphasis is on accurate flying and making sound PIC decisions. There is also quite a bit of work done on instrument flying, so operating on partial panel (suction gyro failure...AI, HSI). The teaching at Stapleford is excellent though and 25 hours of instruction seems to be sufficient to pick all of this up. Time spent on accurate nav and familiarisation of the area you will be learning in I would say is the best use of time.
And finally, enjoy it! It will be a while before you can justify flying around a country to have lunch or go to the beach again!