In order to obtain a graduate place at Oxford, as well as being accepted by the School of Geography and the Environment, a college must also accept you.
On the graduate application form, you may choose to either (i) indicate a college preference or (ii) indicate that you have no college preference, in which case a college will be selected on your behalf. You will not be able to amend your choice after submitting your application. Whether or not you state a college preference will not affect how your department assesses your application and ultimately whether or not they decide to make you an offer.
There are a wide range of colleges in Oxford. They range from the old, traditional colleges with a mix of undergraduates and postgraduates to newer, more informal colleges some of which are only for graduates. Some colleges are large with 300 or more graduate students; others are smaller with only 100 graduate students. Some colleges are located in the centre of Oxford within walking distance of School of Geography and the Environment, whilst others are a cycle-ride away.
Information on choosing a college can be found on the University's Choosing a college webpage.