From the 1981 edition of Oziana, the second story is "The Eldritch Horror of Oz" by Phyllis Ann Karr, illustrated by Sunshine D. Nelson. It's a dark, mysterious story about previously unknown aspects of early Ozian history that, when I first read it, thought was a little weird and creepy, but this time around recognized it as a pastiche to the tales of H. P. Lovecraft! A young Gillikin boy, only an infant when Oz was enchanted, slowly ages and eventually decides to enroll at the Wogglebug's college (here given the name of Mistictonic University—see what Karr did there?) and spends so much time there, he eventually becomes a professor. The strange dreams he's had occasionally throughout his life, about a strange black city and its weird crab-like inhabitants, start becoming more frequent, and Professor Wogglebug suggests he take a sabbatical to clear his head. He does, but ends up at Glinda's reading the Great Book of Records (or the Macronomicon, as it's called in this story). In some of the earliest pages he discovers the truth about the origins of Oz and the strange creatures who created it (no, not Lurline and her band) and their sinister purpose for doing so. As a result, our protagonist becomes one of only a handful who know these secrets, and he must decide what to do about it. But the others who know certainly don't want him to let the secrets out!
I suspect I would appreciate this story even more if I were more familiar with Lovecraft (I've never read any of his stories, just about them). This is definitely a more adult take on Oz, but still remaining part of the Famous Forty. But I think I'm fine just sort of ignoring this story and not worrying about the full implications.