i think Onigashima is actually Big Oars Senior. If oars was little oars jrs dad and little oars is also known as oars III, that implies theres another one even bigger than oars was. And onigashima is styled like a grave, and kaido apparently built the palace inside and on top of it later, adding to what it was originally built as. And Marco said it used to have another name.
i think oars may be related to the emerald city as well. consider that Oars is オーズ in Japanese, technically ōzu with a long/extended O sound, and Oz, like the wizard of oz, is オズ, the exact same characters but with a shorter O. and the emerald city is the capital of the land of Oz in the wizard of Oz. Part of the lore of the wizard of oz novels is that whoever rules the land of Oz also gets renamed Oz. (Or Ozma if theyre a girl).
something else i just noticed from wiki is that emerald city is written with 都 • (miyako) as "city" which is an archaic/ancient word that means "the place of residence of the emperor" and only means "capital city" as an extension of that. by comparison when shandora is called the city of gold it uses 都市 • (toshi) city; town.
so if the Emerald City is an "ancient" place... why not be where "ancient" giants are from? Was Onigashima's old name "Oz"?