I have already dealt with Julia but i am struggling coming up with O'Brien. Would it be something to do with the theme of independance?
Yes, independance, but taken in two very differing ways--a type of "doublethink" if you will. Firstly the relationship between O'Brien and Winston is one of the master-slave variety. O'Brien is the member of the inner-party and is consequently the master in the relationship. O'Brien provides "the book" for Winston and encourages Winston to question the current hegemony to see how far Winston will go. O'Brien asks Winston and Julia during their clandestine meeting (179-83 of the Penguin edition)various probing questions with the intent to find outhow far they're willing to go to subvert the rule of Big Brother. It is all about control here, because O'Brien is influencing and exerting control over Winston and Julia even though he seems to be encouraging their "independence" and free will. Control and manipulation, trust in an unknown Godhead--these are all themes you might discuss in the relationship between Winston and O'Brien.
I'm not sure they have a "personal" relationship ... I don't think that O'Brien is capable of such a realtionship. Winston is a "tool" for O'Brien, a defective citizen who needs to be made to think "correctly." If their "relationship" emphasizes anything, it is that resistence to the Inner Party is futile, that they live in a society where power exercised for its own sake, is normative, and where any human interaction is viewed with suspicion.
this was a question specifically asked in the IGCSE Paper 5 variant 3, just today.