Of course it varies a lot per subject, Littleham - some subjects interview up to 80% of their applicants, others far fewer. And of course the "real" numbers of applicants and places are very different.
The Oxford site describing entry in 2013 said that 57% of all candidates applying to Oxford were shortlisted for interview. Over 36% of those shortlisted for interview received offers of a place at Oxford. So overall the odds are 1 in 3.5 (ish) to get an offer post interview, but again this will vary.
The good thing seems to be that the admissions people appear to be trying very hard to ensure that the right people get the offers, irrespective of the college they applied to.
Good luck to everyone - I've been a parent on both sides of the equation - DD1 read English at Oxford and loved it (though still remarks on the fact that it was the hardest she has ever had to work ever) , two years later DS1 was rejected after interview for PPE and instead went to Durham and loved it (and still remarks on the fact that it was less hard work than school, certainly for the first two years). DD1 is still trying to find her dream job in theatre/TV production, DS1 has a training contract for law already lined up to start in two years after his GDL and LPC exams.
Meanwhile DD2, who I posted about on here this time last year, was invited for interview at Oxford for Human Sciences, having been pushed by school to apply, but turned it down because she and we realised that the preparation and stress would be too much for her (she has suffered from Chronic fatigue for four years). Instead she concentrated on her Alevels, firmed an offer from Durham for Natural Sciences and achieved 3 A s and an A at A2, and an A for her EPQ. Durham were more than happy to defer her offer to 2015, she resisted "suggestions" from school to make another Oxbridge application, and she's having a gap year now which I hope will allow her to put her health first and recover from her CFS, and build up her strength and self-confidence.
Sorry, that was an essay. The message I think is that Oxbridge is really oversubscribed with almost all the applications coming from very bright young people. Not all of them can be accepted - but wherever your DC ends up I am sure it will be the right place for them