Canon

Canon: Whenever an author writes a series, everything he or she wrote themselves is considered canon. To put it more simply, canon is the original material that was stated by the writer in their work.

Manga
To know the story of One Piece and its content, you must look towards the Japanese print of the manga; where the series started from. The manga is the backbone of the One Piece materials, without it, One Piece wouldn't exist. Everything, everyone, any events or information is found in the manga is canon.


 * An example - In the manga, Ace met with Luffy and his crews briefly in Alabasta. After this Ace destroyed the Billion's ship and left. In the anime, Ace stayed on with the Straw Hats until just before they arrived at Yuba.

Fanon
If an unofficial source, say a fan of One Piece, decides to write a story based on Eiichiro Oda's work, then it's Fanfiction.


 * An example -In the manga; Vivi was the princess of Alabasta, that's canon because that is how the writer stated it. If you say that Vivi was a Martian queen, that's considered fanfiction because Eiichiro Oda did not write Vivi to be a Martian queen.

Cross Epoch
This crossover comic that featured the Dragon Ball cast and One Piece cast does not have anything to do with the main storyline, fans don't consider this a canon.

Omake Features
The comics Oda drew in the omake has nothing to do with the canon, despited that Oda drew them. These comics featured the One Piece cast in different roles and different situation which are standalone stories.

Movies
From movie 1-9, they were suppose to take place during the series but they have no connection to the main storyline. These movies were based on the events of the series.


 * Movie 1 takes place in the East Blue.


 * Movie 6 was based on Davy Back Fight, only that it's darker, plus the events are bigger but is nothing like Davy Back Fight.


 * Movie 7 was suppose to take place before Water 7. Since Machines are in the movie, Oda added Machine Island to the canon storyline.


 * Movie 9 takes place in Drum but Vivi is not in it, Robin and Franky makes an appearance while Wapol has an older brother and new subordinates. The canon Drum has nothing like this