Power is a difficult theme to assess in a novella like Of Mice and Men. A more pointed theme is alienation or lonliness. It seems as if, however, most alienation or lonliness in the text comes as a result of someone else exerting their power over another character.
For example, Lennie is under George's power and control. Lennie somehow comprehends that this is for his own good. Ironically, Lennie has enough strength as it relates to physical power to take George out if he wanted to.
Another example of power is when Curley begins beating on Lennie in about the middle of the book. Lennie doesn't fight him back at all... until George lets him. Curley is exerting his power as the boss' son. When George lets Lennie defend himself, all Lennie does is grab Curley's hand and squeeze it. This shows Lennie's physical strength so strongly that the group decides (under Slim's direction) not to say a word about what actually happened, but to report he got it caught in a machine.
These examples of power are linked to other themes because Curley would not have to do what he did except that he lacks the confidence in himself. He is trying to show off. This demonstrates he must have feelings of alienation or he wouldn't go to such great efforts. With George, he uses his power sparingly and in an effort to keep peace. Dealing with Lennie's mental disability he must make these efforts or the two of them would constantly be in trouble.
Of Mice and Men is a novella in which power plays a significant role. One way to talk about power is to see who has it and how it's used, as the post above does. Another way to look at the theme of power in this work is to see who doesn't have it, why they don't, and the consequences of being powerless.
Lennie is powerless in all the ways that matter. He isn't particularly able to think for himself or make his own decisions because of hs mental disability. The consequences of that are being dependent on others for nearly everything and continually being held in check on things he does want to do.
George is powerless in one sense--because he loves Lennie and feels responsible for taking care of him, he is powerless to do the things he'd really like to do.
Candy is powerless now that he's no longer able to be productive. He was once empowered; since his injury, though, he has been powerless, just like his dog, to live what he considers to be a productive life.
Curley's wife is powerless in that she is trapped in an unhappy, loveless marriage but wants nothing more than to be loved. This unfulfilled need drives her to be the lonely "bad girl" she ends up being.
Crooks is powerless because of his color as well as his physical deformity. Society might be able to overlook one or the other, but certainly not both. As a consequence, his inside is as deformed as his outside.
How power is wielded against the powerless, as well as the consequences of being powerless, is a fascinating concept in this novella.