what are the main contradictions in oceania george orwell's 1984

There are many but I'm thinking you might be referring to Big Brother's three slogans. Really, if you examine them, they are really paradoxical. They are, however, meant to seem contradictory.

1. War is peace- While Oceania is at war the party thinks people will not disagree with them. It's the old "either you are with us or against us thing". War is historically a good time keep power.

2. Freedom is slavery- While people are free to do what they will, the country cannot be strong. Everybody must think and feel the same thing, and then the collective people can be strong. If people are divided by individual ideas, they will be conquered.

3. Ignorance is strength- While people do not think for themselves the nation will stay strong. The Party, like the Nazis, are not interested in intellectuals. They want people who do not think, who are ignorant. This way they can fill their little mind receptacles with whatever they want.