Rhetorical devices are persuasive devices. Old Major wants to persuade the other animals that they can have a better future. He wants them to believe they don't have to accept their lives of endless toil.

To do this, he first appeals to his own ethos, or credibility. He reminds them of his great age and that he has had much time to think. These attributes give him authority. He says that for these reasons:

I understand the nature of life on this earth as well as any animal now living.

Building his own authority makes it easier for him to persuade the other animals to accept what he has to say.

Old Major also uses pathos, or emotional appeal, when he says he doesn't think he will live much longer. This encourages the other animals to pay closer attention to his words, because they might not have access to his wisdom much longer.

Old Major leans on logos, or logical appeal, as well. This is the persuasive power of facts and statistics. He states that the resources are there in abundance to support a better life for the animals:

This single farm of ours would support a dozen horses, twenty cows, hundreds of sheep

Old Major persuades, too, by using hyperbolic or emotionally weighted rhetoric when he describes the animals' current plight to appeal to their emotions. They are "forced to work to the last atom of … strength" and treated with "hideous cruelty."

Old Major also employs oversimplification in employing what is called a "black and white" view of the world to make his case. It is either them or us. Humankind is bad and the animals are good. Old Major states:

Man is the only real enemy we have. Remove Man from the scene, and the root cause of hunger and overwork is abolished for ever.

As the novel will show, "Man" is hardly the animals' only enemy, but Old Major's rhetoric gives the animals a common enemy to unite around.

One primary rhetorical literary device Old Major uses is that of repetition. There are many classes of rhetorical repetition. One Major uses often is the repetition of beginning of clauses. This kind of repetition is called anaphora. An example of anaphora is Major's "No animal in England" repetition.

Old Major includes a rhetorical technique called the "call to action" in which he tries to stir the animals to work toward the eventual rebellion. A call to action is often used near the end of a persuasive appeal, once the speaker/writer has stated their case, to get the audience to do something. Old Major does it with the line:

For that day we all must labour

He wants the animals to "labour" in the sense that they will prepare for the revolution, even though it may not occur for a while.

He also references a "dream" in which all creatures would be equal in a world without man. This, of course, would be pathos, as it is fundamentally an emotional appeal to a world that the rest of the book demonstrates is unreachable in reality. But Old Major appeals at least as much to logos, arguing that the removal of man from the equation would make everything better. In this, he mirrors the rhetoric of both the Abbe Sieyes, whose incendiary "What is the Third Estate?" made many of the same arguments with reference to the nobility, and Karl Marx's Communist Manifesto, which argued for the inevitability of proletariat revolution.. 

We usually discuss speeches in terms of the classic persuasive appeals.  These are known as ethos, pathos and logos.  Here’s a quick review: http://courses.durhamtech.edu/perkins/aris.html

Pathos is an appeal to emotion.  Old Major does this a lot when he decries man as the only animal that does not produce, only consumes.  Here is an example of pathos.

No animal in England knows the meaning of happiness or leisure after he is a year old. No animal in England is free. The life of an animal is misery and slavery: that is the plain truth. (ch 1)

Ethos is basically use of character to persuade. Old Major uses his influence as a well-respected old boar to get the other animals to listen to him.  He even tells them he is about to die (which is also pathos).

Now, comrades, what is the nature of this life of ours? Let us face it: our lives are miserable, laborious, and short.

Logos is the use of logic to convince.  Consider the use here.

"Man is the only creature that consumes without producing. He does not give milk, he does not lay eggs, he is too weak to pull the plough, he cannot run fast enough to catch rabbits. 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. Our labour tills the soil, our dung fertilises it, and yet there is not one of us that owns more than his bare skin.  (ch 1)

Very logical, but also very inflammatory.

Citation:

Orwell, George. Animal Farm;. New York: Harcourt, Brace, 1954. Print.

He also appeals to logos and pathos.  He talks about how he is old and has thought about many things.  This is an appeal to logos.  Then he paints a highly emotional picture of what the animals' lives are like right now.  This is an appeal to pathos.

Allusion (or reference) is one major component to Old Major's speech as he refers to the old song which becomes the animals' anthem. 

Old Major also uses rhetorical questions as a device in his speech as well.