Strictly speaking, you can't be a member of "OneWorld", you will be a member of a program belonging to one of the airlines in OneWorld, like British Airways Executive Club, so pick a program with care.
However, there is no need to sign up for any new FF program until you have booked your tickets, it could even be unwise to have an old account if at a later date that program decides on a nice sign-up bonus. (I'm still regretting that I didn't enlist more parents, siblings and whatever for my family account in BMI Diamond Club in 2008(?) when the 9000 miles bonus came...
)