Why does O'Brien appear to befriend Winston? How could Winston end up caring for him, even though he tortures him?

From the very
beginning,is more smitten withthan he is with .  Ironically, he thinks Julia is the
thoughtpolice (and she may have been), but he mistakenly sees O'Brien as a member of the
Brotherhood.

The state has conditioned Winston to betray his own mother and
the future mother of his potential children (Julia), in favor of the fatherland (Big Brother and
O'Brien).  Indeed the boot crushing force of the totalitarian government seduces rebels toward
it.  Winston abandons all that Julia helped him believe about Big Brother in pursuit of O'Brien.
 Winston ultimately trusts the father-figure (O'Brien) over the mother-figure (Julia), such is
the conditioning of the Big Brother.

He mistakenly thinks Julia is a double
agent and that O'Brien is not.  Since the members of the Party are unable to speak or write to
each other, they must conjecture (through physical appearances only) about one's allegiances. 
Throughout the novel, Winston will place more importance on love of O'Brien than of Julia: that
O'Brien's abilility to see truth is more important than Julia's ability to give and receive
love.  He will mistakenly follow the Father(land) instead of the mother.