I am not familiar with O/L exams -- assumed they are like our SAT exams -- but for any exams my advice is three-fold:
1. Find what works for you; it's not one approach fits all as most will try to tell you.
2. Contrary to most advice, I have always found extensive note-taking, under-lining, etc., to be counterproductive for me: I found myself focusing on what I should write a note on, or underline, rather than trying to grasp the concept. (This probably works better for liberal arts subjects than science/math, and as stated may only be the way my quirky memory works.)
3. As others say, don't cram at the last second, or right up to the time of the exam. You reach a point of overload. Build in a pause, to relax, clear the brain's RAM short-term memory that may otherwise overwhelm your understanding of the full question being presented to you. (I think this applies to most people. After all, these types of tests are designed to measure your in-depth knowledge more than immediate problem solving.)
Good luck!
