Uncle Henry represents the "typical" western farmer. He never laughs, works all day, and is depressed.
The yellow brick road represents the gold standard, in the movie it shows the yellow brick road leading to palace, or the power.
The Scarecrow represents the western farmers because he complained about not having a brain but he was the best problem solver out of the group, and in that time western farmers were supposedly stupid.
The Monkeys represent the Native Americans because they used to do what they wanted to do happy and free, but now they are ruled by the settlers. "Dorothy and friends are told when they meet up with the monkeys that they were once a free people who happily lived in the forest where they flew from tree to tree eating fruit and nuts and doing whatever they pleased without having to call anyone master. This was years before Oz appeared from the clouds to rule over the land."