At this point, O'Brien arrives and question Winston, who reveals that he hates Big Brother. What is expecting to accomplish by sending him to room101?

I think that you must be talking about the conversation
between Winston and O'Brien at the end of Book 3, Chapter 4.  At that point, Winston
does say that he hates Big Brother.  When he says this, O'Brien says something that
shows us why he is sending Winston to Room 101.  He tells
Winston

'You
hate him. Good. Then the time has come for you to take the last step. You must love Big
Brother. It is not enough to obey him: you must love
him.'

So Winston is being
sent to Room 101 to completely break his will.  The Party does not just need him to obey
-- they need him to actually believe in the Party and to love Big Brother with all his
heart.  They need agreement, not just obedience.