I'm unsure which part of those three words you don't follow.
You asked how to unlock the phone to all networks - There is no unlocking process needed, as it's already unlocked (as in, it was never locked in the first place) as O2 don't lock any phones they supply, other than iPhones and some Blackberries, and afaik the N900 doesn't even support the possibility of operator locks in the first place.