What weapons other than devil fruit powers and swords could ocs in the one piece world have?
Hey! Thanks for the ask and sorry for my long absence! I had a friend over, things happened, university started, then more things happened and are currently happening, so... I might not be as active for a while.
Anyways, back to this question. The answer is fairly simple: Literally any you can think of. Anything that could even somewhat be considered a weapon can be used by a One Piece character. But let me give you some options to think about:
I'm gonna touch on swords anyways because I feel like people don't fully realize what range of swords there is. Look at this post. It's a meme post, but you get the gist. The most common sword in One Piece is the katana but did you know that there are different lengths of katana? You could also give your OC a Zweihänder, a viking sword, a Roman gladius, a rapier, a cutlass, a cane sword... get creative!
Second very common weapon: Guns. Once again: Get creative! Marine soldiers use rifles, but you could also opt for a revolver, a flintlock, hell, a bazooka, a machinegun...
Different long-range weapons: Consider slingshots, bows, crossbows and the like.
Smaller, stealthier weapons. There are very tiny guns out there. Daggers, shurikens, kunais, needles, war fans, brass knuckles...
Polearms! Spears, lances, bo staffs, halberds...
There's a lot more out there. Whips, axes, hammers, clubs...
Create your own weapons! Literally, get creative. Maybe research some old weapons that are obsolete today - you'll find some really interesting stuff!
And if none of the weapons you find strike your fancy, your OC might just use hand-to-hand combat or martial arts. Or maybe they ride a ferocious animal? There are some big, dangerous animals in the One Piece world, you'll find or come up with something.
Literally, there are pretty much no limits to your creativity when it comes to what weapons to give your OC. My advice would be to think about what kind of weapon would suit or OC's abilities or requirements. A stealthy thief would not carry a Zweihänder because it's big and hard to conceal. Someone who doesn't have a lot of physical strength would not do well with a weapon reliant on brute force.