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.”
Usopp: Africa (South Africa?)
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?