Old Major is the "patriarch" figure of the animal revolution; he represents Karl Marx, the creator of Communism. He uses rhetoric and propaganda techniques to put forth his views on human and animal relations. In his view, humans are exploiting animals for their own greed.
"Man is the only creature that consumes without producing... Yet he is lord of all the animals. He sets them to work, he gives back to them the bare minimum that will prevent them from starving, and the rest he keeps for himself."
(Orwell, Animal Farm, george-orwell.org)
In his view, animals are fundamentally more entitled to the fruits of their own labor than humans are. Since humans take all that animals produce without giving much back, they are essentially slave-masters, and the animals have both the right and the responsibility to revolt and throw the humans out of power. If animals will just work for themselves instead of for the established hierarchy, humans will realize that they cannot keep animals as slaves and be forced to treat them as equals.
Old Major tells the animals that they are unhappy because of the cruelty and greed of man and that they are ''slaves''.