In most cases, it depends on the specific One Piece novel. Some are officially recognized as canon, while others are not.
The determination of whether a One Piece novel is canon varies. If it aligns closely with the main storyline and is sanctioned by the series' authors, it's likely canon. Otherwise, it might be considered supplementary or non-canon material.
It's a bit complicated. Usually, novels that are directly approved and supervised by the creators of One Piece are considered canon. But not all novels fall into this category.