For Oxbridge, GCSEs are part of the application process and weight fairly heavily, so your grades probably will hinder you there, but if the rest of your application is spectacular you might be in with a chance.
Don't resit, it usually looks bad and you'll kick yourself if you resit and still don't get into Oxbridge, it will have been a waste. I'd suggest you apply to Oxbridge just to see but also apply to other unis, which won't weight GCSEs that heavily.