In 1984, what is the significance of the wine which O'Brien serves Winston and Julia?
In Part Two, Chapter Eight of 1984, Winston and Julia go to O'Brien's apartment where they drink wine for the first time in their lives. This act is significant for two reasons: firstly, because it is used to mark their entrance into the Brotherhood, an underground resistance movement. The act of drinking wine demonstrates their commitment to rebelling against the Party and is similar to a religious baptism.
Secondly, as Winston comments, the wine serves as a link to the "vanished" and "romantic" past because it is no longer a part of the lives of ordinary and everyday people. It is, therefore, representative of Winston's innermost desire to remove the totalitarian rule of the Party and to replace it with the democracy of the past.
The primary significance of the wine which O'Brien serves Winston and julia is that it is contraband and only available to inner party members. No one in Oceania would be privvy to drinking wine unless it was illegally procured. Wine might also hold some distant link to the wine used at religious events. O'Brien could be seen as a "high-priest" in that regard.
The final significance of the wine is that they use it to toast Emmanuel Goldstein, the enemy of the party.