O'Brien's place is not referred to as a mansion; it is Winston's apartment building, ironically, that is called Victory Mansion. Its name is a misnomer: it had been a mansion in the 1930s, when it was first built, but it is more like "the projects" now in 1984.
In Chapter 16, O'Brien's place is used to juxtapose Winston's dilapidated Victory Mansion. Whereas Winston guzzles down his Victory Gin, O'Brien sips his wine. Whereas Winston must always hear the news from the telescreen, O'Brien has the power to turn it off. Whereas Winston is always wanting razors or chocolate, O'Brien has good food, tobacco, and a servant at his beck and call.
Here is how O'Brien's place is described. Notice the fear words in bold:
It was only on very rare occasions that one saw inside the dwelling-places of the Inner Party, or even penetrated into the quarter of the town where they lived. The whole atmosphere of the huge block of flats, the richness and spaciousness of everything, the unfamiliar smells of good food and good tobacco, the silent and incredibly rapid lifts sliding up and down, the white-jacketed servants hurrying to and fro — everything was intimidating. Although he had a good pretext for coming here, he was haunted at every step by the fear that a black-uniformed guard would suddenly appear from round the corner, demand his papers, and order him to get out. O’Brien’s servant, however, had admitted the two of them without demur. He was a small, dark- haired man in a white jacket, with a diamond-shaped, completely expressionless face which might have been that of a Chinese. The passage down which he led them was softly carpeted, with cream-papered walls and white wainscoting, all exquisitely clean. That too was intimidating. Winston could not remember ever to have seen a passageway whose walls were not grimy from the contact of human bodies.
Notice, the place is intimidating because it has all that is contraband and taboo and, worst of all, it is clean and spacious! Oh, the humanity!
What we take for granted in the post-WWII free world has the earmarks of royalty according to Winston. It is not ironic that Winston should fear this place for the sheer luxuriousness of it. After all, why would a rebel and thought criminal live here? Winston is too in awe to figure this out.
The encounter with O'Brien here is suspicious for two reasons. So says Enotes editor:
Two factors especially lend a surrealistic quality to the meeting. O’Brien assures Winston that they will definitely meet, “In the place where there is no darkness,” echoing the suggestion introduced earlier. In addition, Winston finally learns the last line of the nursery rhyme, “Oranges and lemons, say the bells of St. Clement’s,” which has so haunted him. He willingly accepts this remnant of an innocent past from O’Brien.