In general, the 'One Piece' anime sticks closely to the manga. But there are times when the anime adds more details or extends certain scenes for better visualization. However, the core plot and character developments remain the same.
The One Piece anime often has some extra content not found in the manga. It also might change the order of events a bit for better animation sequencing. However, the core story and main plot points remain the same.
The main differences in One Piece include some character designs being slightly altered in the anime and some story arcs being condensed or expanded for TV. Also, the anime might have different music and voice acting that can affect the overall feel.