Please don't believe that:
90%- A*
80%- A
70%- B and so on...malarck!!!!
That is for UMS, which is the converted mark. Raw marks (the actual mark you score on your paper) vary each year, depending on difficulty, because they need X percent of people to get A*, and then Y percent of people to get an A, and so on!
You can look at the OCR website for previous grade boundaries, as for some subjects they are quite similar, but others (e.g Science) really vary. But in a short answer, boundaries are never 'set' for each year!