Hey,
First of all, you should ask Oxford (or any other university you're interested in) whether they'd be willing to accept your first degree as an entry qualification (you should send them an email), as this is possible in my knowledge.
With regards to A-Levels, they usually take 2 years to complete, although it is possible to complete them in 1 year (provided you start soon for exams in 2016). Oxford, like the great majority of universities, require 3 A-Levels (I believe Philosophy and Theology requires AAA in these three subjects).
There are over 40 subjects to choose from (ie Maths, Physics, languages, History, Economics, Philosophy etc etc). There are also several boards providing these examinations (all are considered equal for admission purposes), but, since you're an international student, you would want to look into
CIE and
Edexcel, which offer A-Levels internationally. Clicking on any subject will provide you with the specification (which is all the stuff you need to learn, plus some other information), past papers which are essential for practice and some other exam-related information which might be useful.
Be mindful that you should choose exams involving written exams ONLY,
without coursework, as centres rarely accept external candidates for exams involving coursework components. For example, Edexcel's History and English Literature A-Levels involve coursework, which would make it extremely difficult to find a centre, while CIE's History and English Literature A-Levels do not involve coursework, therefore making it much easier. You should check their specification to make sure that only written exams are needed.
Now, onto the A-Levels themselves; Each A-Level is comprised of AS and A2, like you said. AS is comprised of two units and A2 is comprised by two further units (usually people take AS in one year and A2 in a second year). These units, when combined, form the A-Level, and -depending on how well you did- you'll be awarded a grade like A* (top), A, B, C etc.
For example, if we take
Edexcel's Economics A-Level, you'll see that in order to get an A-Level in Economics, you'll need to take all four units in the
specification (ie Unit 1 Competitive Markets — How They Work and Why They Fail // Unit 2 Managing the Economy // Unit 3 Business Economics and Economic Efficiency // Unit 4 The Global Economy).
Taking the first two units will award you an AS, which is not sufficient for entry. Taking the further two units will combine the AS with A2, forming the whole A-Level.
A-Level exams are held in the May-June examination periods, although I believe CIE offers October-November examination periods as well.
Depending on what you're good at (especially if you've already done the subject at university), you should choose an appropriate subject. I suggest that you choose popular subjects that have course-specific student books, which you can find off of Amazon (ie Mathematics).
You should definitely ask your local/national British Council office on advice regarding how to enter the examinations before taking any steps, so you can actually be sure that you can take the exams there.
And yeah... that's it I think.
If you need further advice, do ask (and do not forget to quote)!.
In any case, best of luck!