One common theme is the clash of beliefs. Hidan has his extreme religious beliefs, and the OC often has different values. Their relationship then becomes a battleground for these clashing ideas.
First, you need to develop your OC. Decide on their appearance, personality, and backstory. Are they a shy ninja or a bold fighter? Then, think about how they would interact with Hidan. Since Hidan is a rather wild and unpredictable character, your OC should have some qualities that can either match or contrast with his. For example, if your OC is calm and collected, their interactions with Hidan's impulsiveness can create interesting scenes.
Some Hidan x OC fanfictions could center around a mission they go on together. The OC might be from a different village and is forced to team up with Hidan. Their different values and beliefs would lead to a lot of clashes and also some unexpected moments of understanding. In these stories, the author can really play with their personalities and how they interact with the world around them.
There's also 'Hidan's Unlikely Companion'. In this fanfiction, the OC has a special ability that catches Hidan's attention. It's full of action scenes as they face various enemies together. The writer does a great job of portraying Hidan's unique personality and how it meshes with the OC's.
The OC could be an outcast in the ninja world. They don't belong to any major clan and are constantly on the move. Hidan might see this OC as an easy target. The OC might have some unique skills, like being able to communicate with animals, but these skills are not enough to protect them from Hidan when he decides to sacrifice them for his own dark purposes.
The OC could try to explain to Hidan that they are not a Jashin fan. They should be very clear and firm in their statement. For example, they could say something like 'I have no relation to Jashin, you are mistaken.'
In a 'Hidan Sacrifices OC' fanfiction, the OC could be a sacrifice to stop some greater evil. Hidan, believing in his twisted way that sacrifice is the answer, chooses the OC. The OC might be an innocent bystander or someone who was in the wrong place at the wrong time. However, the sacrifice could have unforeseen consequences. Maybe it angers other characters in the Naruto universe who then seek revenge on Hidan for his cruel act.
Well, it could be about an original character who gets caught up in Hidan's sacrificial rituals. Maybe the OC is a new recruit in the Akatsuki and Hidan decides to use them for his Jashin worship. The OC might have some special power or knowledge that either makes them a prime sacrifice or a target for Hidan's wrath.
If Hidan were to be captured and tortured, Kakuzu would be enraged. After rescuing Hidan, Kakuzu would stay by his side during his recovery. He'd be uncharacteristically gentle, making sure Hidan had food and water. Hidan would be touched by this side of Kakuzu that he rarely sees, and it would strengthen their relationship.
Well, if Hidan mistakes an OC (original character) as a Jashin fan, there could be some wild consequences. He might try to involve the OC in his Jashin - related rituals. Since Hidan is extremely devoted to Jashin, he could see this as an opportunity to either convert the OC or use them as a sacrifice. It would be a really chaotic situation for the OC as Hidan is not one to be reasoned with easily when it comes to his beliefs.