In an SBS question, a fan asked if the Straw Hats were real, what nationality would they be of. Based on their appearances, Oda gave his answer.
D: “Oda-sensei, I have a question. If One Piece was set in the real world, which countries would the nine Straw Hats come from?”
Oda: “Well. I’ll just put what fits with the character’s image.”
The whole crew is supposed to have roots from different heritages. Interestingly the only Straw Hat Member who is inherently Japanese is Zoro. Curiously enough, Usopp doesn’t get his own nationality, but instead we have to guess where in the continent of Africa he might be from—maybe he’s South African?