completely agree with muppet of the north. however, in my preparation I used a couple of books about passing cognitive selection tests, in addition to the normal OASC type examples. I would recommend using these as a complementary source of sharpening your overal mental agility and lateral thinking.
there is no "silver bullet" book out there that will tell you how to pass OASC! just like the current affairs, you have a wide range of sources available to you to help broaden your knowledge/abilities.