Maybe the OC is a skilled healer and Zoro, in his possessive way, ensures that no other pirates can steal the OC away. He sees the importance of having a healer in the crew especially considering the dangerous adventures they go on. He might even train with the OC to protect them better during fights.
One idea could be that the OC has a special power which Zoro becomes possessive over because he realizes its potential in battles. For example, the OC can sense the weak points of enemies, and Zoro wants to keep this ability within the crew for strategic advantages.
Another plot could be that the OC discovers an ancient treasure map. Zoro, being possessive, decides to be the main protector of the OC during the search for the treasure. Along the way, they face many challenges and Zoro's possessiveness grows as he realizes how important the OC has become not just for the treasure but also for the crew's morale.
Another plot could involve a battle where Law and Zoro have to fight against a common enemy. Maybe a new and powerful pirate group emerges that threatens the safety of their respective crews. They start off not trusting each other fully but as the fight progresses, they learn to work together and combine their unique fighting styles. Law could use his Room to manipulate the battlefield while Zoro unleashes his powerful sword attacks from within the Room, creating a deadly combination.
The relationship can be developed by exploring their backstories. If the OC has a tragic past related to pirates, Zoro can relate to it in some way. His possessive attitude can be seen as him trying to prevent the OC from going through more pain. They can also have shared interests like a love for a certain type of food or a particular island's scenery. By focusing on these commonalities and delving into their pasts, the relationship between Zoro and the OC can be richly developed.
One idea could be that during a dangerous adventure, Zoro and Robin get separated from the crew. They have to rely on each other's skills. Zoro's combat abilities and Robin's knowledge of ancient texts help them navigate through an ancient, forgotten island filled with mysteries and traps.
Well, perhaps the OC is from a long - lost tribe that has a connection to a hidden treasure. Corazon, being in the Donquixote Pirates, stumbles upon the OC. At first, there's a lot of mistrust, but as they face various challenges together, like dealing with other pirates who also want the treasure or escaping from the Marines, they start to trust each other more and more. And the OC could also have some knowledge about the Will of D that intrigues Corazon.
Well, an interesting plot could be an OC who was originally a civilian but got forced into CP9 due to some family secrets. This OC could then struggle between following the harsh orders of CP9 and their own moral compass. They might also form an unlikely alliance with some of the Straw Hat Pirates while on a mission, which would put them in a difficult position within CP9.
One idea could be that the Killer OC has a unique devil fruit ability that allows them to control time in a limited area. They could use this to outwit the Straw Hats during an encounter, creating a really intense battle scene.
Another plot could revolve around an OC who is a former victim of Doflamingo's cruel rule. This OC could be on a quest for revenge. They might start by gathering allies who were also affected by Doflamingo and then plan a elaborate scheme to take him down. Along the way, they could face many challenges and obstacles set by Doflamingo and his loyal followers.
In One Piece fanfiction, a Zoro OC could be a navigator. Since Zoro is not great with directions, this OC could have a lot of comical and also meaningful interactions with him. They could bicker about which way to go on the Grand Line, but in the end, the OC's navigation skills help Zoro and the crew reach important islands. Also, an OC could be a historian who travels with Zoro. They would share knowledge about the lost history of the One Piece world, which could be very fascinating for Zoro as he is always looking to become stronger and having more knowledge could contribute to that.
One idea could be creating an OC who is a long - lost relative of a Straw Hat member. For example, an OC who is related to Luffy and has a unique form of Haki that they develop in a different way than Luffy. Another idea is an OC who is a marine with a sense of justice that conflicts with the traditional marine views, leading to interesting storylines within the One Piece world. Also, an OC from a new and mysterious island with a special Devil Fruit power that no one has seen before can add a fresh aspect to the fanfiction.
Well, a great plot might be that the OC is an artist who can draw the secrets of the Poneglyphs without actually reading them. Robin is fascinated by this ability and they work together to uncover the true history of the world. Also, perhaps the OC has a connection to Robin's past, like knowing someone who was important to her family, which creates an emotional bond between them.