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This is complete ********. Oxbridge do not discriminate based on whether someone went to a public or private school. The reason why the proportion of private-school students getting places at Oxbridge is higher than the proportion of students studying at private schools is because private-school educated students get significantly better results than their state-school educated counterparts. And given that your predicted/achieved academic results is one of the most important criteria for admission, it’s no surprise (and also not unfair in any way) that there are a lot of private-school educated students at Oxbridge.</p>
<p>And with regards to interviews, my anecdotal evidence (as well as factual info from Oxford tutors) is that for subjects like Law, E&M, Medicine and PPE, the percentage of applicants invited to interview is around 35-50%. For other, less competitive subjects with no pre-interview test, the proportion getting interviews is significantly higher (whether it’s 70% or 90% is trivial).</p>
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I happen to have quite a bit of data that was accidentally put online for a brief moment a few months ago. In the academic year 2008-09, there were about 20 American (US = Nationality and Country of domicile) newly admitted ugrad students. In total, there were about 80 American ugrad students studying at Oxford during the same year.</p>
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I was at Merton for the E&M interviews and found it to be a fantastic place. 
I’ll be starting there in 2010 (got in with deferred entry). Hopefully you enjoyed your visit and will choose to apply there!</p>