I graduated an Oxford foundation organ scholarship a while ago, and here's my advice.
Don't pursue an organ scholarship to increase your chances of getting into Oxford. An organ scholarship at Oxford makes you the busiest student in your local College (depending on which College you go to, you could be spending more time on your Choir-related duties than on your actual degree, and the degree is already a headache). If you enrol on an Organ Scholarship for any reason other than a complete undying love for playing the organ and working with a choir, you will have a terrible time of it. Organ scholarships aren't hobbies at Oxford - mistakes are not tolerated, you're expected to turn up to Evensong being able to play the prescribed repertoire flawlessly while watching a conductor and listening to a choir. The work, the stress, the sky-high demands and expectations can only be made bearable if the organ is the love of your life. Organ scholarships should really be thought of as jobs, and those at Foundation colleges are very very much full-time jobs on top of your degree. Plus, it doesn't increase your chances of getting in - in fact academic requirements are slightly more stringent on organ scholars in some colleges, because they want to make sure you'll be able to do your degree relatively easily while working the organ.
BUT
Playing the organ, especially when working with a choir, is the best feeling in the world. I went in to Oxford solely to play the organ (it is the standard route for cathedral organists). The life of the organ scholar, while unimaginably busy and straining, is an experience without equal. Nowhere else are you expected to make music to a professional standard every day with one of the best choirs in the country (or the world, depending on your college) while studying. If you love playing the organ and conducting occasionally, and are adequately prepared, it can be the best place in the world.
Basically, I think you're approaching organ scholarships the wrong way around. Don't start learning the organ so you can have a chance of getting into Oxford / Cambridge: start learning the organ because it's the most wonderful instrument to play. If you like it enough you think you could make a career out of playing it, then the organ scholarship is for you - you'll need to play at an excellent standard and you'll want to get experience working with choirs, but if you have the motivation, the passion, and the work ethic, it can be the experience of a lifetime.