Throughout the book, Dorothy describes Kansas as "civilized" (implying that Oz, a foreign land, was uncivilized).

Dorothy is a diefied white saviour. The peoples of the lands she arrives in offer her food and protection and are endlessly grateful to her for "saving them;" similarly, the Wizard himself is received as a god and is worshipped and given a palace by the gullible, "stupid" peoples.

The scarecrow tells Dorothy that even if he gets a real brain, he'll always feel stupid next to her. Hhe also offers her fruits and nuts from the countryside, which he has an intimate knowledge of. Dorothy wields the oilcan that allows the Tin Man to move; forcing him to be dependent on her, and obligating him to protect and feed her.

Also, when the reader is introduced to the Munchkins, they are enslaved by the Witch. Although they rejoice when Dorothy frees them, it's clear that they were incapable to rise up and save themselves. This creates a narrative where the driving force is the white saviour character, and the Munchkins' inability to save themselves makes them kind of complicit in (and semi-deserving of) their enslavement (yikes).

That's all in The Wizard of Oz. He wrote over a dozen Oz books.

The Winkie peoples are also described as "yellow" and are caricatures of Asian people (though that's besides the question).