I've written before about Ozma's father and grandfather, but what about earlier rulers of the land? Pretty much all we know about them is that they were all named either Oz or Ozma. There is a short story by Fred Meyer and Robert Pattrick, "Mr. Thinman in Oz," that mentions Ozma's great-grandfather. He is called Emperor Ozandahan, and he had a magic carpet that somehow ended up in Baghdad. I think there's a story there, but putting that aside for a moment, I'm going to take a look at some other sources, as well as my own imagination, for information on earlier rulers.
First, we have Scott Dickerson's The Magic Book of Oz. While Dickerson followed only the Baum books, and hence his fate for Pastoria doesn't line up with what Ruth Plumly Thompson told us, I see no reason to disregard his earlier history. The first ruler of Oz was a member of Lurline's fairy band named Ozymandius, the Great and Good Knower of What Is to Be. He married a Winkie girl named Yammy, and abdicated in favor of their daughter Ozma Ebrateb once she came of age. Since Yammy was not a fairy, I assume the immortal blood became diluted over time, and the later rulers were not immortal. Ozymandius himself had his spirit accidentally transferred into a wooden canary, but he continued to live.
According to Marcus Mebes' Lurline and the White Ravens and Magic Tapestry, a woman named Oziana eventually took the throne of Oz, and her daughter Ozia married Ozroar, King of Morrow. As I said last week, I suspect his original name was something like Roarer, and he took the "Oz" upon marrying into the Ozian royal family. Despite some attempts to link the two, this Ozroar is not the same as Ozma's grandfather. He is, however, presumably the father of King Evrard of Ev by another woman.
Randy Hoffman's story "Mixed Magic Makes Misery" has the royal family hit with a pickling plague at some time around the thirteenth century, and replaced by a wooden Board of Directors. The Board eventually splinters, and the old dynasty is restored.
Ray Powell's Mister Flint gives a list of old kings: Neillian, Snoward, Plumthon, Cosgrolla, McGrawignera, Gruellan, and Boz. According to Powell's character Zon, the Land of Oz was named after Boz, but this is highly suspect. If Boz is Ozma's grandfather, as Zon indicates, then the account in Dorothy and the Wizard means there were many other rulers named Oz and Ozma before him. This doesn't necessarily contradict Powell's list, although it would have been remarkably prescient for the kings to have names so similar to those of the authors who would later write about them. Gruellan (named after Johnny Gruelle, author of the Raggedy Ann and Andy tales) would have to be the same as Ozandahan, and his son's full name might have been Ozroar Bozkinz (he's called "Ozroar" in Blue Emperor, "Boz" in Mister Flint, and "Oz Bozkinz" in Magic Book, so this name combines the three).
I've estimated this Ozroar's birth as being around 1750. Why? Just a bit of math that probably has a lot of holes in it, but I don't know of any other attempts to calculate this. There are spoilers in this for Paradox, by the way. In that book, we find that the Man Who Lives Backwards, an alternate-universe version of Ozma's grandfather, is about two at the time of Ozma's coronation. And while we don't know when he was captured, I have seen the date of 1826 given for Pastoria's ascension to the throne. Not sure if this date is based on anything in particular, but it's as good as any other, I suppose. Hence, the Man Who Lives Backwards would have been about seventy-six at this point. If Ozroar had to be the same age as his double, that brings us back to 1750 for his birth. Ozroar's wife is Ozara, who was almost enchanted by Ozma but rescued by a time-traveling Button-Bright and the Wizard of Oz in Atticus Gannaway's Wonderful Journey. Both Mister Flint and Dennis Anfuso's Astonishing Tail of the Gump feature searches for Pastoria's wife, who is named Ozette in the former and Cordia in the latter. And according to Donald Abbott's rather suspect How the Wizard Saved Oz, Brigadier-General Riskitt is Pastoria's half-brother, but we aren't told whether they share a father or a mother.
So those are my thoughts. It's all highly speculative, but how could it not be, given how scant the canonical evidence is and how much the apocryphal works contradict each other? I should also point out that some of these rulers might not have ruled much beyond the central green land where the Wizard would build the Emerald City. Any thoughts?