Everyone knows Oda is a master when it comes to foreshadowing, as this can be seen often times throughout the series in many different aspects of the show.
For example, Haki is referenced innumerable times before being officially introduced – from Shanks’ first use of Conqueroor’s Haki against a Sea King to his Conqueror’s Haki clash with Whitebeard, causing shifts in the weather above.
Many people chalk it up to retconning, but upon rewatching One Piece, I caught something amazing in the Drum Kingdom arc. Oda directly references all three forms of Haki, literally describing each one so succinctly and subtly that I’ve never seen any mention of it before!
Exchange between Chopper and Dr. Hiluluk in Episode 85:
Chopper: “Doctor, What’s a pirate?”
Dr. Hiluluk: “A pirate? Pirates are great men of the sea. They have a body hard like stone(Armament), the eyes of an eagle(Observation), and a voice that tears the sky asunder(Conqueror’s)!”
*Theory by ThunderStorm3
