You'll find a detailed discussion of the themes in Orwell's classic dystopia in the "Themes" section of the eNotes study guide…but this book is such a joy it is fun to sum them up again.
Several themes run through this novel; you can state them different ways, but as concepts, they are as follows:
Freedom vs. totalitarianism. Oceania is a total state, or trying to be. They want control of everything, and they use the symbol of Big Brother to work toward this control.
Language vs. reality. See Newspeak and how the government officials are trying to create a language that won't let people think the wrong thoughts.
Despair vs. joy. This shows up in how the government distorts sex, how it sells bad products, but also in what it does in torture and to families.