I did OCR A with the old syllabus which has now kinda finished. The Step up from AS to A2 is quite big. However that is not the hard bit. The hard bit is the exam. The Chemistry A2 exams itself are so hard, that even if you know everything in the textbook, it can still be very confusing in the exam because they give you lots of things that you've never seen before.
At AS I did little work and scraped a D in my F322 mock 5 weeks before the real thing. In the real thing I got a high B.
At A2, I worked really hard and made lots of notes and practiced a lot ! but still ended up with a B (5 ums off an A)...
Proton NMR is a nightmare btw - arguably the hardest topic