Solipsism, at it's essence, is the theory that nothing exists outside of the consciousness of the individual. Basically that reality is inside of the skull. Solipsism is also explored in 1984, albeit not in the classical sense. The solipsism of 1984, as stated by O'brian, is more a collective solipsism, the theory that the external world exists only through conciousness. The party finds this world view effective because if reality is internal, and conciousness is alterable then reality itself is a simple matter to modify to one's liking.
"It was as though some huge force were pressing down ... upon you – something that penetrated inside your skull, battering against your brain, frightening you out of your beliefs, persuading you, almost, to deny the evidence of your senses. In the end the Party would announce that two and two made five, and you would have to believe it. It was inevitable that they should make that claim sooner or later: the logic of their position demanded it. Not merely the validity of experience, but the very existence of external reality, was tacitly denied by their philosophy. The heresy of heresies was common sense. And what was terrifying was not that they would kill you for thinking otherwise, but that they might be right. For, after all, how do we know that two and two make four? Or that the force of gravity works? Or that the past is unchangeable? If both the past and the external world exist only in the mind, and if the mind itself is controllable…what then?"