Ace has not used Observation Haki explicitly in One Piece, but what is clear is that he had the ability to utilize Armament Haki to ... the fact that fans did not get to see much of his journey ...

There are problems with Observation and Armament Haki due to unique abilities and diminishing need. The power system of One Piece is one of ... the characters also have to get stronger to ...

Haki in One Piece post-time skip is more important than ever before, with Oda exploring deeper facets of this power. Advanced Grade Armament Haki ... will most definitely get to see more of ...