Final answer:
O'Brien refers to Winston as the "last man" to emphasize his status as the last remaining individual in a society controlled by the government.
Step-by-step explanation:
In George Orwell's novel 1984, O'Brien refers to Winston as the "last man" as a way to convey his belief that Winston is the last remaining individual in a society completely controlled by the government.
O'Brien sees Winston as the last person who has the ability to question and resist the Party's oppressive regime. He sees Winston as a threat to the Party's power and wants to break him by forcing him to conform to their ideology.
This characterization highlights the theme of individuality versus conformity in the novel and underscores Winston's role as a rebel against the totalitarian state.