Question about reapplying/transfering

Hello! I'm an international student living in London and attending an international school wherein the majority of students graduate and go to university in America. Because of this, I am a bit uninformed and am about to perhaps ask an incredibly obvious question.
This year, I applied to and was rejected by Oxford, but still feel confident that I could get into the university. I have an Unconditional Offer from St. Andrews at the minute but otherwise have not heard back from anywhere else.
My question is...
I really think I want to reapply to Oxford, but my mom will not let me take a gap year without a 100% chance I will have somewhere to go if Oxford rejects me. Is it possible for me to reapply to Oxford whilst at St. Andrews? If I get reapply whilst at St. Andrews, will my place at St. Andrews still be held for the second year? Any information you have regarding this would be REALLY helpful; I am finding a lot of conflicting opinions about this.