Oden’s journal contains a prophecy about how strong pirates will enter the New World and help take down Kaido. Yamato appears to be having an identity crisis, spouting off seemingly inconsistent lines left and right. However, Yamato has not once denied their birth name; what Yamato doesn’t understand is who plays what role in the prophecy.
As the story is unfolding, Yamato is actually trying to understand what’s happening in the context of the prophecy, and we can understand everything they’ve said by stepping through chronologically with this in mind.
- Finding the journal: Yamato witnesses the death of Oden, and believes his family and all of his retainers are also dead. Upon finding the journal, Yamato learns that Oden was supposed to open Wano for Joy Boy’s arrival. Yamato begins to think of “Oden” as the one who fulfills the prophecy in the journal: a free man that helps strong pirates overcome Kaido and open Wano. Yamato thinks that they’re the only one that knows this, and resolves to fulfill the prophecy and “become Oden”.
- Meeting Ace: Yamato meets Ace, and doesn’t mention becoming “Oden” once. After all, nobody needs to be “Oden” until the time of the prophecy. However, Yamato does form a close connection with Ace, and learns all about Luffy.
- Meeting Luffy: Yamato introduces themselves to Luffy, saying “I’ve been waiting for you! I’m Yamato, Kaido’s son!” We’re meant to think that this is “Oden” waiting for Joy Boy, the one from the prophecy. However, Yamato is not sure which if any of the invading pirates is Joy Boy. Instead, they seek out the one they do know: Luffy, Ace’s brother. We know that Luffy is the one Yamato is waiting for from the titles of Chapters 999-1000: “The sake I brewed while waiting for you”, “Straw Hat Luffy”. Yamato even uses the name and title which Ace knew, in the off chance that Ace mentioned Yamato to Luffy.
- Attic conversation: Yamato explains to Luffy that since the Akazaya Samurai are dead, “someone has to carry on Oden’s will”, so they will do it. When Luffy questions this, Yamato reacts by trying to apply the prophecy: maybe Luffy is the free man that will help open Wano, or “more Oden than me”.
- Learning that Momonosuke lives: One of the last things that Oden said before he entered the boiling oil was “it is imperative that I survive”. He believes, from the prophecy, that he is the one that will guide the world to the Dawn. Yamato isn’t able to reconcile this part of the prophecy until they learn that Momonosuke is alive, at which point Yamato realizes Momonosuke plays the role Oden believed was his. Yamato is willing to die for Momonosuke not because they think Momonosuke is their son, but because Momonosuke is that important.