The amount of people called for interviews for PPE somewhat on the college, but the Oxford website says that around 75%-80% of applicants get invited. Also, 1 in 5 get a place, so if you make it to the interviews your odds have indeed risen from 20% to around 27% (it also means that you'll have about 2.7 people against you) - on average, that is. Statistics might depend on the college but they also said they will pool your application (send it to otehr colleges) if the particular college is oversubscribed, so I guess that figure is pretty much it.
I got my interview invitation yesterday by email so obviously I'm happy now. Flying to the UK on Sunday one week from now. Excited.