An A in Biology is not a disappointing result and that is a stellar set of A-Levels which does meet the entry requirements for Biochemistry, so why would you want to apply to another course out of a perceived notion that you'll be more likely to secure an offer for Chemistry at Oxford? That's bad and suggests you're chasing Oxford rather than the course you'd like to do. So, before you decide your next steps, keep in mind that you choose the degree first, then the university, not the other way around.
You should ask yourself, do you have an offer for another university? Do you think you'd be happy there? Do you really want to risk taking a gap year just for the sake of Oxford, with absolutely no guarantees you'll get in this time around? I understand the allure of Oxford, but if you already have a place elsewhere, you can still excel there. By reapplying, you have little to win (just a fancy university) and everything to lose (you might not get it and to your own effects and purposes, wasted a year of your life for nothing). Decide carefully!