How does the Party control the people of Oceania in the novel 1984?

The Party uses surveillance, thought control, endless war, and endless propaganda to control Party members in 1984. 

As we learn early in the novel, the television screen in every one's apartment is not only a way for residents to watch television. Through the screen the Party watches them:

Any sound that Winston made, above the level of a very low whisper, would be picked up by it, moreover, so long as he remained within...

The Party uses surveillance, thought control, endless war, and endless propaganda to control Party members in 1984. 

As we learn early in the novel, the television screen in every one's apartment is not only a way for residents to watch television. Through the screen the Party watches them:

Any sound that Winston made, above the level of a very low whisper, would be picked up by it, moreover, so long as he remained within the field of vision which the metal plaque commanded, he could be seen as well as heard.

The Party also alters history to maintain control. Winston's job is to rewrite old newspaper stories to align with whatever is the so-called "truth" at the moment. In this way, the Party hopes to maintain total control over the truth, which, of course, in its hands, is a lie. More importantly, the Party has been trying to cut down the number of words in the language to a bare minimum. To do so, it is creating a language called Newspeak. It believes that without many words to choose from, people won't be able to formulate thoughts that oppose the Party.

Endless warfare diverts people's attention from the misery of daily life and gives the Party a rationale for maintaining an oppressive police state. As long as there's a war effort to justify it, the Party can demand people work long hours and live on tasteless food rations and Victory gin.

Finally, ceaseless propaganda supports all of this. From the endless posters proclaiming "Big Brother is watching you" to the daily "Two Minute Hate" sessions directed at the "enemy of the people," Goldstein, the Party keeps people's minds full of what it wants them to know. The Hate sessions also target people's aggressions away from the Party and toward other enemies.

Winston can not turn off the television in his apartment. The telescreen can invade people's lives and thoughts at any moment. The following is an example:

The voice from the telescreen paused. A trumpet call, clear and beautiful, floated into the stagnant air. The voice continued raspingly: ’Attention! Your attention, please! A newsflash has this moment arrived from the Malabar front. Our forces in South India have won a glorious victory. 

With such endless distractions, constant surveillance, dumbing down of discourse and propaganda, the state keeps an iron grip on its Party members.