O'Brien is a member of the Inner Party who masquerades as a member of the Brotherhood in order to sniff out rebels. He tricks Winston and Julia into believing he is trustworthy, so they come to him to learn how they can help bring down Big Brother.
In Book Two, Chapter Eight, O'Brien tells the lovers that as members of the Brotherhood, they will have to be willing to give their lives, kill, commit acts which will hurt innocent people, betray their nation to foreign powers, and live life as an undercover agent with a new identity.
Basically, acts of terrorism are what is expected of them. Winston and Julia agree to all of these conditions. However, the one thing they refuse to do is part from one another. Julia and Winston's love is the one thing they refuse to surrender for the good of the Brotherhood.
When Winston makes the allusion to "the place where there is no darkness," O'Brien does not appear surprised that he knows about the phrase nor does he seem ignorant of its meaning.
O'Brien gives Winston and Julia tablets before they leave, then promises to send them a copy of Goldstein's book.