A dilemma attaching to applying for Human Sciences at Oxford is that the course will necessarily be quite different from your other 4 choices, at least a couple of which won't even share the name. That obliges you to wonder how much you want to 'go for it' in your personal statement, something which wouldn't attend an application for, say, English. "My interest in studying English stems from...", does for
everywhere. "My interest in studying human sciences..." will only seem peculiar to the admissions tutor at Bristol who understands this to be an application for the BSc in Biological Anthropology.
A safe path might be to look at Oxford's suggested pre-reading list (it is here:
http://www.ihs.ox.ac.uk/prospective-students/introductory-reading/ ) and choose some several books that you can mention in the PS such that it is clear to Oxford that you're on their page without its being alienatingly obvious to everyone else that in your mind it's Oxford or bust.