In what way (O'Brien) is the domination desired by the "Party" different than the world domination desired by the Nazi's and the early communists?
O'Brien's domination is the perfection of what the Nazi's and early Communists sought. In his world, O'Brien is able to control people (Winston, eg.) to such an extent that external reality ceases to exist and is replaced by whatever the powers in control wish to put in there. This is symbolized by the 2 + 2 = 5 that Winston finally agrees to. O'Brien didn't settle for Winston's agreeing to stop the pain; O'Brien continued until Winston actually believe that it was true. At that point, Winston had given up his hold on what we think of as reality; he could believe anything.
Imagine what the Nazi's could have done if they had this power. They were able to "force" their ideas down the throats of many of the people; the people agreed that 2 + 2 = 5 not because they actually believed it, but because it was what kept them alive. Revolt was always possible because of the link to the "real" world, the knowledge that 2 + 2 still = 4.
O'Brien gives it away when he tells Winston that the difference between the dictatorships of the past and the one in 1984 is that power is not a means in 1984, it is an end. It is exercised in a way that it has never/rarely been exercised in our world.