What are some significant events which occur when Julia and Winston go to O'Brien's, that connect to the theme of this scene?

The primary themes in the novel are as follow:

• The Power of Big Brother;


• Freedom and Enslavement/Free Will;


• Appearances and Reality;


• Loyalty and Betrayal;


• Utopia and Anti-Utopia;


• Patriotism;


• Information Control;


• The Degradation of Language;


• The Triumph of Drudgery

In Part II: Chapter 8 (or Chapter 16 overall), Winston and Julia visit O'Brien's.  The following evens are significant (themes in parentheses):

1.  O'Brien's place is too good to be true (appearance vs. reality, the power of Big Brother):

"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."

2.  Winston meets O'Brien's servant (enslavement):

"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."

3.  O'Brien turns off the telescreen (power of Big Brother):

You can turn it off!' he said. 

'Yes,' said O'Brien, 'we can turn it off. We have that privilege.'

4.  O'Brien presents himself as a double agent, a member of The Party and The Brotherhood (appearance vs. reality):

'We believe that there is some kind of conspiracy, some kind of secret organization working against the Party, and that you are involved in it. We want to join it and work for it. We are enemies of the Party. We disbelieve in the principles of Ingsoc. We are thought-criminals. We are also adulterers. I tell you this because we want to put ourselves at your mercy. If you want us to incriminate ourselves in any other way, we are ready.'

5.  O'Brien gives them a test of allegiance ceremony, and they are prepared to commit atrocities for the Brotherhood; except Julia is not prepared to leave Winston. (loyalty and betrayal)

You are prepared to lose your identity and live out the rest of your life as a waiter or a dock-worker?' 

'Yes.' 

'You are prepared to commit suicide, if and when we order you to do so?' 

'Yes.' 

'You are prepared, the two of you, to separate and never see one another again?' 

'No!' broke in Julia.