In addition to the above answer, for each of his criticisms against man, Old Major backs it up with supporting statements. They are not abstract declarations either, but "hit home" in a very concrete way. Injustice rings out loud and clear when Old Major speaks about the manner in which the animals are overworked, underfed, and even deprived of their young. For the young porkers, they will meet their fate at the chopping block within a year. He offers an arguement difficult to contradict.
Old Major also gives the animals the hope of another viable option. He appeals to an ethic of fairness and solidarity absent under the "regime" of Farmer Jones. This is communism in pure theory, not practice - given in the true Marxist spirit. In reality, it is about as accessible as Sugercandy Mountain.
Through Old Major's speech in Chapter 1, George Orwell satirizes demagogues who address mobs and brainwash them by hijacking their capacity to think and reason. Old Major employs all the tricks of the trade to coax and rouse his captive audience to rebel against Mr.Jones the owner of the farm.
Orwell begins by puncturing Old Major's self-importance by ridiculing him that he was actually once a prize exhibit which went under the fancy name of "Willingdon Beauty." The pronoun "I" is repeated several times in the first pargraph of the speech to emphasise the over sized ego of Old Major who is a reputed rabble rouser who is in love with his own voice.
In order to win the support of his animal audience he lists out all the hardships they suffer under human exploitation which will ultimately end in their death, "to that horror we all must come-cows, pigs hens sheep everyone." Notice the poetic inversion, instead of the prosaic, "we must all come to that horror"- so typical of a soap-box orator. The irony, ofcourse, is that the fate with which he threatens the others does not overtake him, instead he is given a decent funeral.
Orwell satirises a demagogue's penchant for oversimplifying serious issues by reducing them into a slogan, "all men are enemies, all animals are comrades" and the laughable "all animals are equal."
Old Major at the end of his speech gains complete control over his listeners with his 'politically correct anthem.'