US qualifications on Oxford's website:
Successful candidates would typically have an excellent High School record supplemented by SAT I scores of at least 1,400 in Critical Reading and Mathematics and preferably also 700 or more in the Writing Paper, giving a combined score of at least 2,100, or ACT with a score of at least 32 out of 36. They would also expect three Advanced Placement tests in appropriate subjects with two at grade 5 and one at grade 4 or SAT II two or three appropriate subjects at 700 or better.
Candidates must show evidence of an appropriate educational background and good standards of literacy and numeracy. All applications are considered carefully on their individual merits and tutors consider academic achievements, predicted grades, candidates’ personal statements and academic references as well as any written work or written test that may be required as part of the application.
Tutors are much more interested in candidates’ overall academic ability, and their potential and motivation for the course at Oxford, than the particular programmes which have been followed at school. Most people applying will be very bright and have high grades as competition is strong so you should do something to make yourself stand out. Also remember that you would need to attend at least one interview. My best advice for you is to really try your best in school and try to enjoy it so hopefully you will get something out of it.
Hope this helps, good luck. :-)