Currently doing groundschool. Instructors top notch. Can't fault Oxford one bit. Best tip though is have a good look around at all the schools and have a chat with the students. Ask them what is wrong with the place (don't expect 5 star accomodation at OAT!) and you'll have a decent idea about what you can expect if and when you get there. Find out what is best for you. Whilst overall Oxford is probably best in the country it might not necesarily suit you as well as some of the other places for one reason or another.
Good luck!