The major conflict usually discussed in Animal Farm is between Snowball and Napoleon, so this is an interesting question. Just so we are both on the same page, remember that "conflict" describes characters in the story who either have incompatible goals or some underlying psychological division...not always a physical conflict.
Napoleon and Old Major don't have a direct, active conflict in the way that we most think of. Old Major was the initiator of the revolution in that he was the one who initially put the idea into the other animals' heads. He was a catalyst, but notice that he didn't act until he was very old, used up, and had nothing to lose. Old Major doesn't even take part in the actual "revolution" on the farm.
In the terms of the Russian Revolution, Old Major could be seen as sort of a Karl Marx. He's an Idea Pig. He dies before the revolution but he sort of gets the ball rolling.
Napoleon is more like a Joseph Stalin. He takes the ideas of the revolution, as given by Old Major, and exploits them. Napoleon is not afraid to get his hands dirty.
The conflict between them would be seen as more subtle; Napoleon is in almost constant conflict with the ideals that Old Major talked about. The farm animals have come to believe in those ideals and they measure events on the farm by them (in their own, limited way.) So the conflict between Old Major and Napoleon is one of ideas.
The Russian Revolution is the same way. People didn't get involved just to set Stalin up as the dictator of the Soviet Union. He had to twist the energy of the revolution to his own ends, and had to constantly combat the effects of being measured against Marx's original ideas.