It's Libertarian in some sense but I vaguely assume Oda has communist sympathies because he's got che guevara hanging up in his office and the fact that the revolutionaries exist in his story.
Like Luffy's dream is being the most free in the seas, but naturally there are other pirates who value freedom that will fuck other people up like Blackbeard or Kidd. Plus Oda's shown us good natured Marines like Garp, Fujitora, Smoker, and several others. There obviously will be some form of government when One Piece comes to an end, it just won't be at the beck and call of bourgeoisie fascists like the celestial dragons. One Piece is a good baby's first leftism, I appreciate having it when I was younger, it helped me get here.
(fun fact, in this sbs answer Oda makes it a point to say that his stereotypical thoughts are "world peace")