I've just picked up a new recording of Life Begins at 8:40, a Broadway revue from 1936. At first this doesn't seem to have any Oz relevance, but it warrants a closer look!
Life Begins at 8:40 was written by Harold Arlen and Yip Harburg, with some lyric assistance from Ira Gershwin. Of course Arlen and Harburg went on to write the songs for MGM's 1939 The Wizard of Oz. This review from 1936 is intriguing for bringing the songwriting team together with Ray Bolger and Bert Lahr, who both starred in the review, and later in the Oz movie. So, a number of the future Oz elements were combined 3 years earlier!
This also gave the songwriting team some experience in writing for their stars - in particular, Bert Lahr. One of the songs in the revue is Things, which parodies an operatic baritone - on this recording, it's performed by Brad Oscar. It's easy to see this as a precursor to If I Were King of the Forest! Harold Arlen later declared that no one could write better for Lahr than Harburg and himself.