Why is obrien's famous equation important to the book?

I think you are referring to 2+2=5. This really speaks to the idea that 2+2 can equal anything the party wants. While the Party insists on people believing the truth, the truth becomes malleable. The party can make anyone believe anything. Truth is simply what one is told. Any obvious differences in what are and what the party says becomes a moot point. It is the party that is always right; two plus two made five. Next week two plus two could make six or seven hundred and fifty two, it really doesn't matter.