What is Winston’s opinion of O’Brien?

This really depends on where in the novel you are referring to. Initially Winston has an inkling that Obrien is different from the inner party. He begins to project all his fantasies of rebellion onto O'Brien. In reality O'Brrien is interested in Winston but only for Winston's intellect and disposition. Winston becomes O'Brien's "project" for re-education. The result is O'Brrien becoming Winston's torturer and saviour. At one-point O'Brien wants to show Winston what is left of the humanity that Winston purports is still alive. Winston looks in the mirror and does not see his former self. He has become emaciated creature that has rotted beyond recognition. O'Brien pulls Winston's remaining tooth out as if it were a baby tooth on its last strand. Winston becomes a metaphor of the human spirit that O'Brien says has long since lost its meaning.