The advice from BJack about how to approach your PS is good. I would add, in answer to the question in your heading, that the PS is not very important for any Oxford course. Certainly it creates a bad impression if it is poorly written, but you should be able to run it by people for whom English is their first language for readability. But very, very few personal statements really stand out (and it's much easier to stand out by being awful than by being good) so don't worry that the PS is going to be your undoing because it's not.