What is One Piece doing to keep it one chapter/episode? Depending on how condensed the plot is in a manga, the number of episodes that come out annually will be affected. For example, Haikyuu!! had 25 episodes per season for the first two seasons, and since it has almost caught up, there will probably be 10-12 episodes per year from now on each covering 4-5 chapters, as 50 episodes got them to about chapter 150, if they want to stay at pace with the manga.

In an action-based manga where a lot of the scenes are fighting scenes, it becomes pretty hard to lengthen a chapter time-wise. What are specific time-filling techniques One Piece using to keep the chapter-to-episode ratio at 1:1?
The One Piece manga and anime aren't in a 1:1 ratio. The distance beetween them even shifts around. The second arc took place between the 8th and 24th chapters in the manga, while the same in the anime took place between the 4th and 8th episode.

The main reason why the anime is this slow are the fillers. They help the manga "catch a breath" and sneak in couple chapters. (side note: This is still no excuse for fillers in the middle of an arc though).

Oda likes to put in a few filler episodes after every arc or when there's a new movie out.

With these however, there's still a very big chance that the anime will eventually reach the manga. If this happens, there are 4 possibilities:


They stop airing one-piece for a year so the manga catches up.
They start airing the episodes every 2 weeks.
They do what Game of Thrones will do / did and continue with a separate story line.
They start writing the story for the anime and make the manga based on that.