This is what I should have done, and am now advising you to do:-
1. Once you're sure you have picked the right subject, look at Oxford's official website/prospectus to weed out the colleges that don't offer it.
2. Now research the colleges that are left. There is an abundance of information for this: college websites, college prospectuses, alternative prospectuses, TSR etc. You should be able to ascertain the feel of each one, the variations between them and to again weed out colleges. For example, with the course I applied for, EP, I automatically discounted LMH because its section on entry reqs was much more detailed (ie stricter) than say, Worcester's. Also the tutors for each college may be something to take into account. If you're interested in the Chinese economy from the second half of the 20th Century til now, and find an Econs/Pol tutor whose interests match this, then that may be a college you should flag up for serious consideration. There are other factors too- like size, reputation for your subject, distances from certain places, whether it is big on sports/music/drama etc.
3. Visit the remaining colleges. There really is no substitute for going up there and seeing them for your self. Even if you can't, at least make sure you've picked one you really like the sound of, one that you could rattle off a whole list of reasons for if asked "Why did you choose us?"
BUT at the end of the day, the college you choose might not be the one where you are interviewed, or ultimately offered a place at (hopefully you will get to this stage!) so try not to pin all your hopes on College X or Y, or Oxford for that matter. Good luck!