So since nobody really appeared to explain this, the British model of medical education (followed in the UK, India, Australia, around much of the world) is very different from the American model.
When you apply to an undergrad university program for medicine, you're actually applying to med school. It's usually a 6 year program that combines pre-clinical and clinical training. At Oxford it seems they do 3 years pre-clinical and 3 years clinical, although I think at other universities they may do 4+2. At the end of it all you get a degree that's
usually called an MBBS (but at Oxford they call it BM BCh for no other reason than they're the oldest university in the world and they can do whatever they feel like). That degree is equivalent to the US MD (or DO for that matter). After your degree you spend time as a "junior" doctor, followed by other funny titles like "registrar" and "consultant". That's your residency and fellowship. As others have said, if you want to practice in the US, you have to complete a US residency program, and as a foreign medical graduate you will be at a disadvantage compared to American grads (even if you went to Oxford).
Needless to say, competition is intense for UK med school admission even if you
weren't looking at Oxford. Competitive secondary school grads over there typically complete what are called A-Levels in a number of subjects, equivalent to AP exams (but actually a little bit harder). If you haven't done A-Levels in all the sciences then I don't think you'll get into Oxford.
If you're absolutely sure you want to be a doctor then you should start volunteering and getting straight-As, and knocking out some AP courses. Get leadership positions. Apply to some of these programs listed here:
Medical Schools Offering Combined Undergraduate/MD Programs
My uncle was a Doogie Howser back in the 70s. The University of Michigan used to have a fast-track to medicine which was 2 years of undergrad work followed by admission to their 4 year med program, he got into that right out of high school. He's loved his career as a physician and has no regrets, except for the fact that he doesn't technically have a bachelor's degree which has occasionally caused complications.