The average number of A*s at GCSE for a successful Oxford Medical applicant is around 10A*s so you'd find it a challenge based on those grades alone. However, if you come from a school which doesn't usually do well at GCSE then you will get a contextual bonus which will boost your application.
Regardless, if you're serious about applying for Oxbridge Medicine then I would recommend considering Cambridge more so. Cambridge have a higher entry requirement but interview more candidates. Oxford on the other hand look entirely at BMAT score and GCSEs to shortlist candidates for interview.
There are some really useful stats online which I would recommend looking at to help in your decision making. Look at the Medical Sciences Division for OU to find out the breakdown of admissions statistics and you will see.