I don't know what an AS or an A2 level is, although I've seen it in a few textbooks.
Basically an AO level is an exam level. It's kind of a bridge between the O-levels (GCSEs in the UK) and the A-level. Most people don't do AO levels except in GP (which is compulsory for all students*) and in Second Language. Basically Second Language is compulsory at O-level; some people do Higher Mother Tongue and some don't. Those who don't do HMT (or who did it and didn't get a satisfactory grade at O-level) have to do a year of Second Language at junior college level and then take an AO level exam at the end of the first year; if they fail that they have to retake it at the end of the second year. I took regular Second Language rather than Higher at O-level (I got an A2) I took mine last year, got a B3, and decided I didn't want to retake it. The grading system for AO level is similar to O-level, so instead of the regular A, B, C, D and so on there's A1-F9.
*They've recently changed this to an altogether more confusing points-based system for junior colleges. They've introduced a new A-level subject (not AO level), Knowledge & Inquiry which can be done in lieu of GP. But this is only available to students in JC1 from 2006 onwards, so I'm not eligible.