Consider the logic of O’Brien’s comments. Has Winston acted according to his own free will, or has his fate been predetermined from the beginning?
When Winston accuses O’Brien of reducing him to a wretched state, O’Brien argues that Winston himself is responsible: “This is what you accepted when you first set yourself up against the Party. It was all contained in that first act. Nothing has happened that you did not foresee.”
In a way, both. Winston did act of his own free will in choosing to set himself "up against the party," and his fate was determined by that choice. It is ironic in a way, because the focus of the novel is on the concept of control. Winston wants desperately to have the free will to love and think and act as he chooses - to control his own life - yet that "free" will is lost to him. He sees the signs that say "Big Brother is watching you" and knows the "thought police" might suspect him at any moment, and that the "telescreens" monitor his every move. Winston bemoans the disappearance of his family members, talks at length about people disappearing, and yet, still chooses to exert his free will to initially write in his journal. He knew he may suffer the same fate, but could not resist the most basic human desire to control some aspect of his own life.