So you're thinking about applying to Oxford? In that case you have to chose a college. Well actually that's a bit of lie, you don't have to, but 80% of applicants chose to. If your application doesn't specify a college, this is called making an "open application", more information below.
Bear in mind that the Oxford colleges are more similar than they are different. Every college is rather cool, and wherever they end up, students are usually very loyal to their college. Ultimately college choice is not something to worry about. So what is a college? What's the difference between them? How do I choose?
All about open applications
So what happens if you don't chose a college (making an "open application" instead)? You are assigned a college by the central university computer. The college that you are randomly allocated to does not know that you made an open application. Allocation is often to colleges with fewer applicants that year for your subject (this doesn't make them bad colleges, application numbers fluctuate a lot each year). Making an open application does not disadvantage your chance of getting into Oxford (remember the people considering your application don't know that you made an open application) so don't be afraid about making one if that's how you feel. Some people mistakenly believe that their chances of getting in will be improved if they make an open application.
Make an open application if you really don't mind about what your college life is like. However, college life is such a great and unique aspect of Oxford that it's well worth utilising the choice given to you. If there are any colleges you know you really don't want to be at, don't push your luck with an open application... just randomly pick one of the ones you don't mind going to! You could decide right now to make an open application (and save yourself a little time) but you probably want to glance at the rest of the article first to see what choosing would involve.
How to make the choice
Before you start, you probably want to write out (or print!) a list of ALL the colleges. Looks like a long list? Don't worry, the next section will show you how to instantly cross off a few...
First eliminate colleges that won't consider you!
There are a few postgraduate colleges that only take people who have already studied their subject for three years at university. If you don't know if you're a postgraduate, then 99.9% of the time you won't be - you can cross off Green Templeton, Kellogg, Linacre, Nuffield, St Antony's, St Cross and Wolfson colleges. (Note: if you are a postgraduate, you can apply to the other colleges as well as these few.) Everyone can cross off All Souls because they're crazy and don't actually have any students of any kind. Also, if you're not aged 21 or over, you can cross off Harris Manchester because they are a "mature" college. There's no age restrictions for the other colleges.
Next up: not every college offers every course. It's vital that you go check out this page on the Oxford website for details of which colleges offer your course. For subjects with lots of students (e.g. PPE or Maths) most colleges offer the degree. On the other hand, for smaller courses, your choice is made easier (e.g. Materials Science applicants have only got seven colleges to look at). Note that colleges don't usually specialise in certain subject areas: if a college isn't good for a subject, they simply don't take applicants for it.
P.S. Oxford colleges admit students of all genders.
The good reasons
"Its really conveniently located" or "It's right next to my department"
Oxford is a small city with the university dominating the city centre. This means you can walk almost everywhere. Even the furthest away colleges are closer than the accommodation at many other city universities! (Cycling is popular in Oxford if you fancy saving time, but for most people it's not entirely necessary.)
But there's no denying that some colleges are more central than others. Take a look at a map of the colleges: consider where your lectures will be, where the city centre shops are, where the supermarkets are, etc. As a very broad generalisation, location is a good indicator of how old/pretty the buildings look. Colleges in the city centres are usually the old looking colleges, whereas more modern colleges are usually a short walk out of town. But often the colleges on the edges have significantly larger grounds.
If you manage to visit the colleges, you will get a feel as to where some of the colleges are, including walking distances! But here is a map of colleges to help you eliminate those you feel are just a little too out of your way. You might want to be in the centre of the town, you might prefer to be slightly out of the way.
"The accommodation there is superb!"
The most obvious question with accommodation is how many years the college provides it for. All colleges guarantee undergraduates accommodation for your first year. After that there's a variety: some colleges give you accommodation every year (sometimes on the main site, sometimes in annexes), but others have limited rooms so you'll have to hit the private housing market (sometimes having to decide who to live with after knowing people only a few months, although students at other universities manage it!).
The location of college provided accommodation (see above) is probably important - some colleges, or their annexes, can be slightly distant (i.e. most people will cycle rather than walk). You might care about kitchen facilities (sometimes rooms have no access to kitchens), number of people to a bathroom (rarely an issue, although ensuites are fairly rare) and room size (although Oxford rooms tend to be larger than at other unis).
"It has cheap accommodation and food" or "It's ridiculously rich"
The cost of rooms varies a lot between colleges. The cheaper your rent, the more money you'll have to spend on other things. Typically, a richer college is able to make things cheaper for its students. (Be careful when you compare prices online, colleges often have fixed/facilities/catering charges that you have to pay on top of your rent. Also be aware that a small number of colleges require their students to pay for dinner every night. Or lack of kitchens might force you to.)
Also, the richer colleges may have better stocked libraries, better sports facilities, can afford to give you more contact hours, etc. You might want to investigate grants for people who do well in exams, for travel in the vacations, for buying books, for sporting success, for musical achievement, etc.
"It has a particular sports/music/etc facility that I'd use"
At this stage it becomes useful to point out the Alternative Prospectus provided by students are each university. You'll find students' takes on each of their own colleges (good and sometimes bad points). This allows you to find colleges which have a particular facility (such as sports facility, music facility, 24/7 library). It can be hard to find out exactly what each college has - if you're not sure, you can ask current students on the TSR forums. Obviously don't expect every college to have everything, only cross off colleges that don't have a facility that is absolutely essential.
No matter how workaholic you are, everyone has time to get involved with some other activities. There are always university clubs, and you can get involved in activities at other colleges, but having a way of getting involving in college is fun. Consider also the facilities available. Table tennis tables and badminton courts are rare, for example, and having your own sports grounds right next to college is a real plus for sporty types.
"I fancy being part of a small/large college, or at least one with a few other people studying my course"
A couple of colleges have around 300 or less undergraduates, but the biggest (St Catherine's) has 500. This may not sound like a huge difference, but it has some effect on whether it is possible to know everyone in your year. Perhaps more importantly, you probably want there to be a few others studying your subject - being the only student at your college studying Music in your year will be quite a different experience to having five others. Note that the physical size of college sites varies quite a lot, and often doesn't match the number of students (this affects how many years you can live on the main site).
"I walked in and it just felt right" or "I like the look of the place"
You want to be happy with the choice you've made. Sometimes people walk into colleges and find they instantly love or hate a college. If you've got a long list of possible colleges then visiting might be a good way to quickly reduce your list. An inspiring environment might help you work and enjoy yourself (and is always good for showing to friends/family). But ultimately the looks of the buildings aren't going to make that much of a difference to your time. However when you leave, it may be the visual memories that stay with you the longest - it would be nice to have memories of a place you loved to be, whether it's because the architecture is to your taste, or because you thought the gardens were pretty.
You don't need to book an open day to visit a college: most are open to prospective students everyday. However, an open day is useful to get a tour in of a college and meet current students (although TSR lets you do the latter from your very own home). Open days also tend to include question and answer sessions and sometimes chances to meet tutors.
... the bad reasons (you can use these, but don't place too much emphasis on them) ...
"It's got an excellent tutor for one of the options I want to take"
Chances are, a considerable amount of your teaching over the course of your degree will be by a tutor in another college. This is especially the case with optional courses, where tutors specialise and only one or two tutors in the university may teach a given option. What's more, you can ask to be taught by a specific tutor if you so wish. So the college you choose will not necessarily determine who teaches you. That said, some colleges are better than others at helping you get the tutor you want.
"I've heard it's a friendly place, and I'm worried about making friends"
They're all friendly places, trust us. When this many people are thrown together into so close an environment, you cannot help but get to know the people around you. It's pretty much guaranteed that you'll make friends wherever you go.
It's worth thinking about the size of the college (see above). And if you're a graduate, think about whether you'd rather be in a graduate-only college or a mixed college.
"The food's meant to be good"
It's true that the average standard of food varies quite considerably between colleges. But the standard of food varies quite considerably between different days at any given college, too! It's not going to kill you at any of the colleges, and if you don't like it, you don't have to eat it. However, if the college doesn't provide self-catering facilities, you might want to be a little careful.
"My best friend's decided to apply there"
It's great to stay in touch with your old friends - but you'll definitely be making new ones too, so don't feel you have to be in the same college as your best mate to keep in touch them with them. Knowing nobody can force yourself to make new friends. If you go to a different college, you can have your own friends, and make a new circle of friends at your friend's college! All Oxford colleges are pretty close, so being able to see each other won't be problematic.
"It organises fantastic entertainment!"
Okay, so your college will probably be the centre of your social life. But note the "probably". If your college isn't known for its bops and balls, go to someone else's. However, the quality of the facilities does matter. After a hard day's work everyone needs to chill out, and if the bar's so dull nobody ever goes there, or if the JCR has nothing but chairs and some old magazines, social life will suffer for it.
... and the ugly reasons (definitely don't use these)
"It's undersubscribed" or "I'm more likely to get in"
Your choice of college does not affect your chances of getting a place somewhere at the university. If you are good enough, you'll be offered a place somewhere, even if it's not at the college you originally chose. Over 20% of current students did not apply to the college they are now at. Choose the college you like most!
"My role model/teacher/careers adviser went there in the Sixties..."
David Cameron isn't Prime Minister because he went to Brasenose. Your achievements in life will be your own, and you won't emulate someone else's by following them physically. Be wary of taking advice from old Oxonians - not only has Oxford changed a lot since they were here, but they'll have a biased view anyway.
"It's much more famous"
Great, so when you tell your aunt you're going to Oxford, she might have heard of your college. But once you actually get to Oxford, fame means only one thing: tourists. After the tenth camera-toting American has knocked on your door asking if they can look round your bedroom, you'll wish you were somewhere a little more obscure. What's more, the death of the old boys' network means your college doesn't mean a thing these days when it comes to getting a job afterwards.
"I've heard it's good for state schoolers"
If you've heard this, someone has misled you. It's not like some colleges make it really easy for state school pupils to get in, and others only admit people with double barrelled surnames.
And once you're a student, it really doesn't matter. You'll go through your whole course without finding out whether some of your friends are from a state or independent school - and if it does come up, only as a curiosity.
"It's high on the Norrington table this year"
The Norrington table is a rough way of judging a college's academic standing. It can be worth looking at the Norrington table over a number of years to see whether a college is generally high or low. Given its problems of measurement and its variability, however, it would be folly to use the table as the sole measure, or to look only at a single year. If in doubt, stay well away.
By the end you'll probably have around four colleges left. Just pick the one you like most. If you sit there saying "I like them all equally" you're probably kidding yourself, there will be one which you like more than the others, so just pick it. Stop thinking about any other factors, if you have that feeling about a college, just go for it. Remember that practically everyone loves their college (and that's despite over 25% of students ending up at a college which they didn't choose).
If you've got questions, you can contact students from each college in the forums where we have a thread for each college. If you've got more general questions, or want to chat to other people who are choosing, check out the Choosing an Oxford College thread.