The onsite accom is in the main Uni style bed rooms and sharing with assorted young idiots. And bloody expensive for what it is.
Just wear what ever you like. But personally I would suggest chino's and a shirt (but not some uniform rubbish) its more to do with getting your professional head on, frame of mind and all that. But its really not required.
In reality your modular most people there will look down on you as scum and inferior to the intergrated students. They won't have much to do with you. But hold no doudt's if you then go on to get a job you will be listed as a great success story for oxford.