Oxford will have the same PS as all the other universities sent to them by UCAS. On the Oxford Application Form, which you also have to fill in, you have the opportunity to write a small, supplementary statement which only Oxford will see. It isn't intended to be completely different/contradicting to your UCAS PS. The key word is that it is a SUPPLEMENTARY.
The only real opportunity you get to distinguish yourself to Oxford above the others is in the interview, if you get one, or any work you are asked to send in, or exam you are asked to sit (in my case, I did the HAT)
The only university I applied for anything history related at was Oxford, so my personal statement was dominated by things relevant to the courses I'd applied for elsewhere (politics and international relations). I ended up having about three lines related to history but I must have done it in the right way because in the end they offered me a place for history rather than the combined honours for which I'd applied.
the additional box can be quite useful if say you`re applying for politics everywhere else and PPE at oxford to talk about why you`re also interested in phil and econ. But otherwise people just leave it blank, generally.
Yeah we were told at school not to write anything on it. Also at a presentation thing I went to with an Oxford and a Cambridge admission tutor they said the same thing, basically unless you have something really important to write then don't bother, because they hate all the drivel about why you think oxford is such an amazing place etc...
And yeah you have to make it one of your five choices because it still needs to go through ucas.