O’Brien is trying to force the Party’s own logic on Winston; more specifically, he is trying to control and reshape Winston’s reality—what he knows to be true—into whatever will suit the purpose of the Party. If Winston has the right to insist on his version of the truth—which is a truth that stands independent of ideology—then he can hold on some vestige of being human and determine at least that much of his life for himself. “You believe reality is something objective,” says O’Brien to Winston, but “reality …only in the mind of the Party…Whatever the Party holds to be truth is truth.” For Winston, if two plus two equals four, then reality is objective, and if this is true he can still seek objective truths and discount the double think of the Party.
In a society like Oceania, whatever they tell you is what you believe. Not the actual reality. If they tell you that 2+2=5, then it is 5 apparently, but really it is 4. If you have the freedom to believe that 2+2=4, then you can believe in almost anything.
In a society where it is wrong, heinous, criminal to agree that 2+2=5, the ability to openly state 2+2=4 shows freedom of thought and individualism which if allowed for, opens up many more choices of one's individual desire.
In Oceania the opportunity to think and do what is considered normal by society at large, was not the norm. This was a society that pretty much brainwashed the citizens to believe anything that they said or did regardless of whether people actually knew what was right or wrong , that what was said or done by the party, was right.
So to actually say that two plus two makes four and knowing that by saying it, thinking it and believing it is right without having to suffer any repurcussions in Winston's eye sight means having freedom. Winston also believed that if one could believe what they deemed was right, and nothing happened to them because of their beliefs, then they had achieved freedom.