1984 essentially assumes that all people are spineless idiots who can be conned into anything if you push them hard enough
Interesting choice of word.
Oceania is built on doublethink, and EVERYONE, from the poorest prole to Big Brother himself, have to doublethink, or the entire system collaspes. Even the Inner Party have to hold contradictory ideas, believing that Oceania is both a great utopia and that it's a cynical way for the rich to keep power; that Oceania can easily annihilate its external enemies, and that Oceania has no real external enemies, only internal. It's not just the Middle Party that are being oppressed, even the Inner Party has to keep on living in a prison (both metaphorically and literally...the shortages ensured that even the richest people in Oceania have spartan living conditions) and worse, the Inner Party consent to terrible living standards and keeps on doublethinking, in the hope of feeling the joy of power and glory and being the boot that crushes the human race.
And, really, Goldstein (or is it O' Brien?) made it clear that's it really the Outer Party that suffers the short end of the stick in Oceania. The Proles are free, just like animals are free. They do not pose a threat to the government. But the Outer Party, the Middle Class, are the people who have enough power and motive to launch a movement to overthrow the Inner Party, and this is why the Inner Party oppresses the Outer Party.