Oh, and patent leather shoes, if possible.
OP, I think this would give you a good idea re the variety of dresses that are acceptable as black tie: http://www.telegraph.co.uk/fashion/s...k-tie-dresses/
I think strictly speaking, heels are part of black tie attire but in practice, you probably can get away with some nice flats. I used to walk/ cycle to events in flats because walking around Oxford in heels is a real pain, and carry my heels in a bag. That might be an option worth exploring, and I recommend keeping a pair of flats on hand if you're going to any event that requires you to stand for some time (eg balls)
More generally, I would say bring at least one black tie dress when you go up so that you don't have to rush around looking for one in your first week! I brought 3, and that suited me just fine for 4 years (rewore them quite a few times lol), but YMMV. I also have 1 white tie dress, but it isn't necessary since white tie events are relatively uncommon, and you can probably rent a dress for those occasions. Finally, I would recommend bringing perhaps 2-3 more formal-ish but not black tie dresses since some events require the female equivalent of a suit and tie (eg formals at certain colleges).
If you need to buy clothes, Oxford does have a decent range of shops where you can get pretty much everything, and in any case, there's online shopping (eg ASOS).