I also applied to medicine at oxford this year (and unfortunately was rejected). I applied to exeter college and also had interviews at new college - so those are quite similar to Lucy's...
None of the colleges are too noisy, and they all have quite secluded parts/gardens, so i wouldn't worry about that. Exeter is medium-sized, although physically quite small, but it has lovely gardens and is right in the middle of oxford - it is in a side street off the high street, you couldn't get more central.
On a more pessimistic note wigman15, don't get too set on oxford as there is a very real chance you won't get in (i fell in love with oxford probably a bit too much, and hadn't prepared myself fully of the upset of not getting in). If you look good on paper (good GCSEs etc) its better to apply to oxford than cambridge, as at oxford 70% of candidates are rejected before interview. However bear in mind that despite having brilliant GCSEs, good BMAT results, UCAS form etc, you may still not get in.
Don't choose your college based on the selection procedure as ALL the colleges have a very rigorous selection procedure, and whilst i may not have had things thrown at me, the interviews were very tough, and will be at any college you apply to.
Anyway, good luck!