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
Most of the comics Oda drew in the omake have nothing to do with the canon, despite the fact that Oda drew them. The comics often humorously featured the One Piece cast in different roles and different situations which are standalone stories. However, the second omake called One Piece Report Time supposedly chronicles the Strawhat crew's eating habits during the first parts of their voyage.

Movies
Movies 1-9 were supposed to take place during the series but they have few, if any, connections to the main storyline. Rather, these movies were based on the events of the series.


 * Movie 1 takes place in the East Blue.


 * Movie 3 has Tony Tony Chopper in it, but can not occur in canon since neither Nefertari Vivi (who was there when he joined) nor Nico Robin (who joined the crew immediately after leaving Alabasta and Vivi) are in this.


 * Movie 6 was based on Davy Back Fight, only that the setting is darker, and the events bigger.


 * Movie 7 was supposed to take place before Water 7. Although there are machines in the movie, Oda later added Machine Island to the canon storyline, although the only mechanics on this island were the Spaceys created by Moonwatcher.


 * Movie 9 takes place in Drum, but Vivi is not in it, Robin and Franky make an appearance, and Wapol has an older brother and new subordinates. It is dramatically different from the canon events that occured at Drum.