The original story of the Wizard of Oz was a call to agrarian populism and a critique of wealth concentration. It has a lot to say about gold and silver, too.
Discussion
I think we all learned that in high school and as much as I wish that was what it's about, I think thats an after the fact kinda thing, not the original authors reason for writing.
The movie and story behind Wicked are separate from the original story. I like the interpretation.