Old Major proposes a solution to the animals' desperate plight on Manor Farm under the Jones administration (representing the Tsar and autocracy) and he inspires thoughts of a rebellion of sorts among the animals. The actual time of the revolt is unsaid; it could be tomorrow or several generations down the road. Old Major's "Barn-Yard Speech" at the very onset of the story could be a reference to the Communist Manifesto. Old Major dies shortly after delivering his speech and the animals, stirred up by this speech, set to work immediately on the bringing about the Russian revolution.
Shortly after his death, the animals rise in revolt and oust the humans from power. This rebellious act is so quick that many don't realize it happened until it is over. The animals drive Jones and the farmhands off of the farm and remove many of the implements of his rule.
The Seven Commandments that Snowball had transcribed, that were supposed to encompass Old Major's general philosophy, are gradually altered and deformed under Napoleon until they come to entirely different meanings than were originally intended. Also, "Beasts of England", the song that came to Old Major in his dream, was later banned on Animal Farm by Napoleon, at which time it was replaced by "Comrade Napoleon", a hymn composed by Minimus the pig that pledges allegiance to Animal Farm and to work to protect it.
In both film adaptations, Major dies while provoking the animals into rebelling. In the 1954 adaption, he dies suddenly while the animals are singing. The 1999 version is even more unfaithful- Jones slips in mud while investigating the sounds coming from the barn, fires his shotgun, and indirectly hits Major, killing him.
With Animal Farm being parallel to the formation of the Soviet Union, Old Major was based on both Lenin and Marx. The animals hold him in high esteem, and dig up his skull and walk past it and salute it every day, much as Lenin's body was preserved and is kept on display in Moscow. Towards the end of the story Napoleon announces that he had buried the skull. Marx, author of the Communist Manifesto, died before the first communist revolution, much as Old Major, founder of Animalism, dies before the Animal Farm revolution. His body was saluted by the soldiers every day, even after the rebellion. This show of respect was a sign that the animals remembered their roots and the roots of the Rebellion. Old Major was very much honored in this way because he was the idealist behind the Rebellion and initiated the work towards it.