In deciding whether or not to reapply, one could ask for feedback however this may turn out to be disappointingly generic and consequently meaningless, eg .. there were many applicants and the strength of the competition meant we were unable to offer all strong candidates a place.
I dont think Oxford looks at whether it is a second application. They consider each application on its own merits. I cant imagine they dig up last years application to compare it so they can see whether to shortlist you.
I would recommend contacting the the head of admissions after your results to find out what success gap year applicants had in applying with the gcse grades and as grades and a2 grades that you have and how this compares with last years year 13 applicant. From this, you will probably find out that your chances are lower than the average candidate unless you end up with superb grades.
Go to the subject open day and ask searching questions about how to improve your chances.
As the previous poster has stated, there just aren't enough places and the best candidates go to other great universities. Each college only takes 4 applicants.
I would say if you have an offer from somewhere you are happy to go, its not worth having a gap year.
Finding gap year work that isn't waitressing/ shop work isn't easy. Most work is more long term and employers aren't interested in training someone for the short term. If you don't mind doing this/and have the money to go travelling, think you would do a lot better in the interview, sure give it a go.