We know from Marco that Onigashima used to have a different name. We also saw that Kaido is able to move the entire island with his flame clouds. It almost seems that the entire Onigashima is actually Kaido’s ship, more or less like Thriller Bark was Moria’s ship (an entire island). At this point it’s clear that the Thriller Bark Arc was heavily foreshadowing Wano and all the “Vs Yonko Saga” in general. So I think Onigashima’s current location is not its original one. So where does Onigashima come from?
From what I’ve seen there are two options which seem plausible. First one: Onigashima and God Valley are related and maybe Onigashima comes from here. It’s possible, but I don’t think we have enough information. We know almost nothing about God Valley.
The other option, a better one in my opinion, is that Onigashima originated from “Kaido’s favourite island”, a mysterious island that we’ve seen in Caribou cover story and in some panels before entering the New World (Drake fought against Scotch, a subordinate of Kaido).
But even if that’s the case we still don’t have a name for “Kaido’s favourite island”. If the key to understand the secret of Onigashima is in its name, it would be strange to set up expectation for a name that we haven’t even heard once in the story. For this reason I think it would be more impactful if we already knew the name of the original location, or even better if we’ve already been there with the Strawhats and we haven’t even realized it.
If you’d ask me “What is your favourite reveal in One Piece?” I would need to think about it. But one that comes to mind is the Skypiea reveal. When Nami finally puts together the map of Jaya and Skypiea and realizes that they were once the same island.
I thought it was brilliant, because it was such a simple solution yet it was almost impossible to guess it in advance. It requires us to look from above, at an island level, while we are small and lack the wider perspective. I’m saying this because it could be use as a precedent and the same “puzzle” logic could be applied in this case.
We’ve seen that Onigashima is basically a circle, with a front and a rear entrance (this rear entrance by the way has no reason to exist…). So can we find in some island we’ve already seen a missing circle shaped piece?
Yes! It’s Punk Hazard! If we look Punk Hazard from above we see the central lake connected to the sea via two canals. It is almost a perfect fit. This would also justify the existence of the rear entrance. But basically the reason why I believe this, it’s because it’s a doughnut shaped island with a missing hole inside. And Onigashima seems to be a cool way to close that hole.
The first thing we see before entering Punk Hazard is a plate with three logos: the Marine logo, the World Government logo and a “Oni skull” logo, that resembles in a peculiar way the giant skull in Onigashima. So there is a link between the Oni race and Punk Hazard. We got confirmation of that in chapter 989, when Big Mom sees the Numbers in Wano: apparently Kaido bought them from Punk Hazard.
When the Strawhats entered the island, they saw a giant skull (bigger than a giant) burning in flames and a few panels after that they saw an actual dragon. The dragon foreshadows that Kaido will be the main antagonist of the saga and his transformation. But what is that skull doing on Punk Hazard?Punk Hazard might have been the birth place of the Oni race (or the continent pullers), from which Oars, Oars Jr. and (maybe) Kaido descend. I think it’s not impossible for Onigashima (a giant Oni skull) to be originally from this place.
The island magnetism (why the log pose does not work)
When the Strawhats entered the New World, they had three possible islands to choose from. One of them was Raijin Island (the Island of Thunder). If the climate on Punk Hazard is the result of two awakened devil fruits clashing, it is possible that the peculiar weather of Raijin is the result of the use of an awakened Goro Goro no Mi (not Enel, maybe a previous user). If that’s the case, the extreme weather condition is not the cause of the disappearance of the magnetic field of the island. Something else must have happened.
We don’t know enough about island magnetism, it has not been explained properly. I guess we’ll have to wait until we reach Laugh Tale. But is it possible that the “heart” of the island was extracted and placed somewhere else? (Almost like Law “shambled” the hearts of many people during the arc, but that might be a bit of a stretch.)
It’s also interesting that the only islands we know that cannot be reach via Log Pose are Laugh Tale, Zou (but it’s a creature) and Punk Hazard. There might be something else here…
The major problem with this theory
In two instances we got an explanation on the origin of the central lake. The first one is in the story. When the Strawhats meet Brownbeard, he says something like: “It’s the spot where Akainu and Aokiji’s final clash happened”. So this seems to disprove the entire theory.
The second explanation is in the One Piece 15th Anniversary: Dive to Grand World Minibook, where it is said : “A hole opens due to the two of them clashing. Due to being frozen, the ground cracks on the island and the sea water floods in.”
There’s really not much to say about that, the story has spoken. But we also need to understand that Brownbeard is not exactly the most reliable source in the verse (he was being manipulated by Caesar at this point). Also, databook and other “external sources” have been wrong in the past, like when Shanks was said to have been a Yonko before meeting Luffy in the Vivre Card Databook Volume 1 (a pretty important info if you ask me).
Theory by goombas95 (https://www.reddit.com/user/goombas95)