I'm sorry I can't give subject-specific advice, I'm sure someone else will be able to. Since no one else has responded yet I'll just say this:
With 4 As at AS level (and what looks like some fantastic stuff to put in you PS) you are most probably going to get your second choice Uni even if you don't get an offer from Oxford. So I see no reason why you shouldn't apply - I don't think it would be particularly risky. Obviously the GCSEs aren't amazing by their standards but they're not terrible either and whilst there's this idea that Oxford care very much for GCSEs I think the TSA is what's really going to be key to your application. As you probably know you need to take the TSA if you're applying for PPL so you may want to take a look at some questions/past papers online (they're all available for free if you Google it) and ask yourself if you think you could do well on it. A good TSA score is much more impressive than good GCSEs. I know students with 10 A*s who didn't get interviews presumably due to their TSA score. Not the most encouraging information but you know what I mean. Do well on that and I don't see why you wouldn't be able to get an interview - and then it's just about demonstrating your passion to study the subject (which it looks like you have) and your academic potential and 'teachability'. Basically, go for it - what have you got to lose!
I wish you the best of luck, and if it doesn't go your way there are plenty of excellent Unis for psychology and I'm sure you'll excel whatever you do.
Oh and one last bit of advice - don't let worrying/preparing for and Oxford application interfere with your school work or your general happiness.