The Wizard of Oz is an excellent example of the difference between plot and theme, it even sounds familiar - I've often used it.
[/I]The the theme is what the story is about: "Crime." "Jealousy." "War." "Love."
What you are referring to as "theme," James Frey (yes that James Frey) calls the premise.
The premise is what the reader takes away from the book. It's the "message, the "lesson." It's the reason the writer chose that theme, because she believes passionately in what she has to say about it. It's something that the world needs to know, or to be reminded of, and it's universal: true for all people. (In that guise, Wizard is a poor example - for some people, the best place is anywhere but home, but we won't quibble). "Crime doesn't pay," is a premise. "Anyone who kills a child under 14 should automatically get the death penalty," is another one. "Love plus plenty of money makes for a good marriage." The list is endless.
This passionate belief in the premise and the conviction that the world needs to know or be reminded of it, alone has the power to keep a writer going through the months or years it takes to complete a book. To do it right is mostly hard, lonely, drudgery during which time you sit and type, no matter how you feel, because it needs to be done. And you are the best one to do it.
If your story grows out of your characters, it has a storyline, not a plot. It's character-driven, like Crime and Punishment, To Kill a Mockingbird and many others. The events of the story could be replaced by other, like events, and not affect the story, which is about the characters. Character is not plot, it's storyline.
A story in which the characters could be replaced by other characters without harming the story in any way (Wizard, Alice in Wonderland), on the other hand, calls for a plot - it is plot-driven. Thrillers are examples of plot-driven novels, as are mysteries.
A few authors can blend the two effectively, but they are pros.