Be careful not to confuse political ideologies.
Fascism is different from the Marxist beliefs of Stalin explored in Orwell's Animal Farm and here. Facists believed that they represented a middle ground between the collectives of Marxism and the individual greed of capitalism. Totalitarianism is perhaps the better term to describe the political ideology that Orwell seems to be cautioning against. The lack of free will in a totalitarian state is perhaps the strongest of the political themes in the novel. Big Brother seems to be modeled on Stalin.

Another theme would be the contrast between the attempt to create a perfect society (utopia) and the result (dystopia).

The use and manipulation of language is one of Orwell's recurring themes in his essays and novels.

These themes are explained in more detail in the enotes link provided below.