To fully enjoy the movies, it's best to have a basic understanding of the One Piece universe and its characters. You don't need to be an expert, but knowing the main characters and their personalities will definitely enhance your viewing experience. Plus, it's always more fun to watch with some background knowledge!
The movies are essentially side stories that take place within the One Piece timeline, but they're not directly connected to the main storyline. This means you can watch them as standalone films, but keep in mind that some characters and plot points might be referenced from the main series.
