My DD started at Oxford two weeks ago and is having her matriculation ceremony this morning :) I can't tell you how proud and happy we are, and she is so far having a lovely time.
There's a long Oxbridge thread on here from 2-3 years ago with a lot of advice and support, and one from last year that didn't go well - mentioning Oxbridge really seems to piss some people off on here (I got a bit of a roasting myself a couple of weeks ago for saying DD was going.)
The application makes for a a bit of a rollercoaster year, The Student Room Oxford and Cambridge Forums is your very best friend, all the info we needed was on there. DD's comp only send an occasional student, but they did do a practice interview with her. Also Look up OXAPP on youtube, useful videos about every stage of the process.
The stress starts to build when interview dates are given out, then in January the offer/rejection letters (Oh my God, the letter... waiting for that post...) but I think the very worst bit is waiting for the results, although thankfully we had an AAA offer, I think Cambridge now wants at least one A and Oxford wants it for a lot of subjects. With no January exams next year, I don't think it's possible to be confident of an A before you get your results in August.
DD agreed that it would be much, much better to be rejected post interview, (like most people are) when the disappointment wouldn't be too major and you could get on with focussing on your other choices, than to miss your offer in August. Quite a few missed offers on The Student Rooms forums and the poor kids seemed quite heartbroken.
Good luck to all the applicants, and my advice is not to let them focus too much on Oxbridge, but to try and manage their expectations and big up their other uni choices as much as possible. (Easier said. DD wanted Oxford and no amount of my raving about Durham was going to make a difference, but it's worth a try)