I just got an oxford offer last week. I do very few extra curriculars and wrote about 3 lines about all of them in my PS. None of that was discussed at interview.
I would reccomend doing an Extended project if your school runs it- I was asked about it in both of my interviews at oxford so I guess they found it quite impressive.
Basicaly, get good grades and write a good PS to give yourself a chance at interview, where you just have to give it your best shot.