In general, the One Piece manga English version is quite faithful. There might be some changes in translation to make it more understandable for English readers, but the core story and characters remain intact. Some names and terms might also be adapted to fit the English language context, but these alterations are usually minor and don't affect the overall plot.
The One Piece manga English adaptation is known for its relatively close adherence to the source. It tries to maintain the essence and charm of the original. However, there could be slight differences in how certain expressions or cultural references are presented to make it more accessible to an English-speaking audience.