I am currently studying with the Open University, and I know this post is five years after this thread last saw any activity, but I'd like to clarify that the percentage of your grade requires completely on the type of course you are studying.
I require 40% on my OCAS (TMA essays) and then 40% for my OES (Exam) on one module, but on another I need 30% OCAS and 50% OES.
To forward Niel.A.H's point...
I am studying BA(Hons) Humanities: Literature and Classics, and in my first module I am averaging 88.85% on my OCAS, which would be a first class honours if it were in year 2 or 3. For my OES, I am predicted a score of 75% (my results are still pending). Even so, with pass marks of 88.85% and 75%, my degree is still only classified as a Pass, not a Distinction, meaning that you must achieve above 85% on both areas, not just overall or in one area, in order to obtain a first class degree.