Am I making the right decision? Watch
Srsly, as you will appreciate, it's no different to asking the same question of any uni. Or any life decision, really. You have to accept that you can never really know. Perhaps the best you can do for now is consider the unique features of your Oxford course and see if they seem like a good fit. I'd suggest:
1. In practice, all uni courses will be different from the prospectus. But is there anything in the Oxford one that really concerns or excites you?
2. Academically, the special feature of Oxford is the fact that you will be required to complete, and be quizzed about, a piece of written work, every week. In addition to public exams, you will probably have 2 -4 college exams over the course of your stay. So there is little leeway for taking things easy. After 3 years, you will feel thoroughly tested and have had lots of practice of assimilating large quantities of data and producing results to tight deadlines.
3. You will be surrounded by high achievers, all of whom are used to working hard. So you won't be the smartest person in the class any more, and it can feel both stimulating and daunting. Similarly, there aren't really many people at Oxford who swan around doing no work and watching daytime TV. This is not always a good thing.
4. Your social life will be based around a college that is much smaller than the campuses of most other unis. It is very difficult to be anonymous at Oxford, or to disappear into the crowd. This can feel both comforting and supportive, exposing and oppressive.
5. Oxford is a compact city, and even outside of work, pretty much everything you do will be found within a short walk or bike-ride of your college - pubs and clubs, concerts, sports, punting on the river, woodland rambles - it all happens in the shadow of the university. It's one of the reasons that I suspect Oxonians don't suffer too much from missing out on a social life
6. It's quite a diverse and cosmopolitan city, but of course, nothing like as much as London, Manchester or B'ham
No matter where you choose to go, you will wonder about "the road not taken". But that's life...good luck anyway