I got an A in my O Level Chem (edexcel).
Chemistry, unlike Maths and somethimes Physics, has a lot of stuff to be studied. The AL syllabus is a bit different from the OL syllabus (im not sure about GCSE). You dont hear some words like electrolysis again.
I'm doing the Edexcel syllabus, and most of the calculation part is studied in the first unit and is easy when you do AL maths too. Organic chemistry is probably the largest section in AS level because once started it never seems to end. Other sections are not too big and much more interesting because they are done in more detail, like bonding, energetics and redox reactions.