In response to Plantsitter, I would like to say that it is not true that there is as much class difference as you say at Oxford. Over half the students there are from Comprehensives.
"Oxford is on course to have the highest ever proportion of state school pupils in its undergraduate intake this autumn, publishing figures on Friday showing that just 41.5% of offers were made to private school candidates." (The Guardian 11 March 2011)
Of those who do come from private schools, many of which will be city day schools, a good many may have had scholarships or bursaries at their private school, or come from families whose incomes are by no means as high as might be imagined. They are not simply "upper class". Moreover, once there, Oxford gives substantial financial help to students from families on low incomes, to the extent that those students might be better off than many students from higher paid professional families in the middle income bracket.
The interview must indeed be terrifying, and it is a great shame you suffered as you did; but, in general, tutors try to help draw out pupils from comprehensives during the interview. I am not a university tutor at Oxford but have known several personally who have talked about this.
To susie4 who is disappointed her son has not gone there, it is understandable that you feel disappointed, but remember that so many people in the same position as your son, who go on to other good universities, get on wonderfully well and truly thrive in them. Encourage your child no matter what and do not show your disappointment. There is simply not enough room at Oxford for every clever child - think of medical students, for example. Everyone who applies will have the similar abilities, but only something like one in twenty can have a place.