Not sure about it. I did not do o level during secondary school. I did my local official exam (spm since i am from malaysia) which is equivalent to o level. Our additional maths is similar to as level so i pretty much aced the as level scoring 117/125 in raw marks 95% in my statement of result. as level for me is easy since i have already learned like 80% of the content and the questions asked are less tricky than spm. However a2 is hell for me. I had to learn alot of new things in 6 months. Plus all the stuff you learn in as like the calculus, vector, trigometric etc are much more in depth and the question asked are much harder and required more thinking since they are not direct. I have friends coming from private school who did o levels. They say a level for them is like a continuation for o level. For me as is like revision for my local exam, a2 is like the continuation.