Hi. I’m a successful reapplicant to Oxford (albeit for Chemistry, rather than materials science).
If you take a gap year in order to reapply, then you will absolutely have to decline any offers you get now. Oxford will not accept applications from anyone who has already stared a degree elsewhere, nor do they accept transfers.
What you should do in your gap year (and in the meantime) if you are dead-set on reapplying, is make sure you take up more supercurriculars (e.g participate in relevant competitions like the chemistry Olympiad), try mentoring other students and get practice interviews (so as to get used to articulating your thoughts out loud).
It’s now too late to request feedback if you haven’t already, but if you do have feedback, use it as a guideline as to where your weakpoints are.