Luffy made a cameo in DBZ before One Piece was even released 
A Chinese net user uploaded a panel from what’s supposedly an oldDragon Ball manga. In it, you can see the back of a straw hat (kinda like Luffy’s) and a skinny character in a tank top (kinda like Luffy). This seems to be the only place that this character pops up.
Here’s the catch: the original Dragon Ball, where this was apparently discovered, ran until 1995. One Piece didn’t start until 1997.
So did this panel influence Oda? Not sure. Maybe. Or maybe it’s just a coincidence.
A Chinese net user uploaded a panel from what’s supposedly an oldDragon Ball manga. In it, you can see the back of a straw hat (kinda like Luffy’s) and a skinny character in a tank top (kinda like Luffy). This seems to be the only place that this character pops up.
Here’s the catch: the original Dragon Ball, where this was apparently discovered, ran until 1995. One Piece didn’t start until 1997.
So did this panel influence Oda? Not sure. Maybe. Or maybe it’s just a coincidence.