I was fortunate enough to get an offer, and had resolved to try again next year if I were not successful. With apologies for being the (lone?) dissenting voice, I would not have made an open application. While it is true that this means the candidate is allocated to an under-subscribed college for the subject, it does not follow that this increases the chances of a place, because you will be thrown up against excellent candidates who have been pooled pre- or post-interview to the same college. The overall quality of interviewees is such that the possibility of them being slightly weaker because their college of choice allowed them to be pooled (and there are many reasons for a pooling decision) is hardly worth weighing in the balance. In fact, it is quite common to see successful pooled candidates inadvertently 'trade up' in terms of perceived college prestige.
College choice is one of the few areas where you have some degree of control over the interview process. You can research culture and ethos; for example, I applied to one that had a long history and proven record of accepting a high proportion of state school students. This is particularly important if you have an unusual qualification or educational path; has the college accepted people like you, or people that you may know? There are ways to find this out - TSR for instance!
You can also choose a college such that you will be fairly sure of who your interviewers will be (in at least one of your interviews). They can then be Googled to death to wring out as much useful information as possible about their academic interests and opinions, which can prove useful (if carried out subtly and sparingly) at interview.
Finally, you can make sure that you choose a college in which you know that you will feel comfortable; as a prospective candidate, you have wide access to visit, attend open days, and perhaps even stay overnight (some of the colleges do b&b, out of term) before making your preferred college decison. This may make you more relaxed at interview, and your genuine enthusiasm for the college may shine through, whereas if a college is allocated to you through an open app, it may be one that does not quite suit your personality: too big, too small, too new, too old, too quiet, too scary etc. However slight, this sense of unease may show through at interview.
These are tiny factors I know, but I worked on the basis that most of us will either get into Oxford, or be rejected, by a hair's breadth; relatively few would dazzle at interview (certainly not myself!) and relatively few would be judged academically below par. Of course, all this planning could still result in being reallocated pre-interview! But you are at least maximising your limited opportunities to control your fate.
Sorry to throw the cat amongst the pigeons at this late stage! Very best of luck anyway