Copyright status of The Wizard of Oz and related works in the United States
"The Wonderful Wizard of Oz" was published in 1900, and was the first to enter the public domain in "1956". This allowed Reilly & Lee, the publisher of all the other Oz books (as well as Baum's other books) to issue their own edition of "The Wizard of Oz", which had previously been published by George M. Hill Company and later by the Bobbs Merill Company after Hill went bankrupt..
The Ozmapolitan of Oz
The word "Ozmopolitan" was first used in 1904, in promotional material created by Baum's publisher Reilly & Britton. The idea was that the Wizard of Oz started an Oz newspaper so titled (a conceit that Martin adopts for his novel). Reilly & Britton issued press releases in this Oz-newspaper form in 1904 and 1905, publicizing Baum's "The Marvelous Land of Oz" and related projects. It is possible though not certain that Baum himself wrote some of these early "Ozmapolitan" press releases. The publisher (under its later name Reilly & Lee) issued more "Ozmapolitan" press releases in the 1920s, to publicize the novels of Ruth Plumly Thompson, and in the 1960s for other products (including "Merry Go Round in Oz", with illustrations by Dick Martin). Three decades later, Hungry Tiger Press used the same publicity technique for issues of its "Oz-story Magazine".