The One Piece Movies are self-contained animated films based on the series. They have completely original plots with animation of higher quality than what the weekly anime allows for.
Movies 1-3 and 11, being the shortest, were not released as their own films, but were instead released as double-features with other anime movies.
The animation in Movie 6 was experimental. Much of the world was produced in CGI and was similar to that of Gunbound. However it was used again, albeit to a lesser extent, for Movie 9, and the character design was used for Episode 388.
Like with many anime movies, the One Piece Movie 7 featured scenes in the preview animations that never actually were seen in the movie itself. These included Sanji fighting a raptor-like mecha (which was changed to a man in a machine) and Zoro's fight against one of the machines was much longer. While the other One Piece movies also suffered such scene cuts, Movie 7 was noted for having the most noticeable cuts.
Movies 8 and 9 are both based on story arcs of the anime/manga of One Piece. Episode of Alabasta: The Desert Princess and the Pirates is based on the Alabasta Arc and Episode of Chopper Plus: Bloom in Winter, Miracle Sakura is based on the Drum Island Arc.
Movie 4's ending follows directly into Movie 5's opening, making these films the only ones to be linked.
So far, only Movies 8, 10, 12 and 13 have been dubbed by FUNimation, and there were no movies dubbed by 4Kids. However, movies 1-7 and 9 will receive a subtitled-only English release in the UK from Manga Entertainment.
Movie 11 was the first One Piece film to be animated in CGI, and the first film to be in 3D.
Three of the movies, 10, 12 and 13, are referred to as 'films'. These movies were directly overseen by the creator of One Piece, Eiichiro Oda.