you would have to check with the individual universities, but generally, A/AS level is really used for admission and althogh A/AS tend to give you credits when you do get in, all you need is decent A-level grades to get in, and from there you decide what you want to do.
Canadian Universities, as a genereral guide, look for 3 A levels and about 4 O levels (where the A levels studied were also studied at O level as well) and where C and up is considered. Or 2 A levels and 5 O levels. (2AS=1A, for the purposes of admission)
The best bet is to check the requirements of your country for admission into a Canadian school. For individual programs you should check anyway. (chemistry at O-level is often sufficient, but once again check).
btw ...*i think* for medical studies, once you have good grades in anything, you will be able to get into pre-med, which would be your portal to whatever you wish to study in medicine.