Well, family is a huge theme. The OC brother becomes part of the Winchester family in a way. They go through the same struggles as a real family, like protecting each other from demons and other supernatural threats. Also, discovery is a theme. The OC discovers his own powers or his place in the world of the Winchesters. For example, in some fanfictions, the OC brother realizes he has a special ability that can help in their hunts, and it's all about how he comes to terms with it and uses it for the good of the group.
One common theme is redemption. The OC often has a past they need to make up for, and Castiel helps them in that journey. For example, in many fics, the OC might have accidentally caused harm in the past and is now seeking a way to right their wrongs with Castiel's guidance.
Love and sacrifice are also prevalent themes. OC angels may fall in love with humans or other supernatural beings, which is usually forbidden. But they are willing to sacrifice their status, powers, or even their lives for that love. This creates a lot of emotional drama and makes for some really engaging storylines in the fanfiction.
Power struggles are also quite common. Lucifer and his brother often have different levels of power, and in the oc (original character) fanfictions, the oc might get involved in these power dynamics, either trying to balance them or being affected by them. For example, the oc could be a mediator between the two brothers in a power struggle. And sometimes, there are themes of redemption, where either Lucifer or his brother is on a journey to redeem themselves, and the oc plays a role in that journey.
A common theme is the struggle between the normal world and the supernatural world. The OC girlfriend often has to adapt to the dangerous and strange world of hunting. For example, in many fics, she might start as an ordinary person and then learn how to fight demons and monsters.
One common theme is protection. The OC older brother is often shown protecting Naruto from harm, whether it's from the villagers' hatred or from dangerous enemies. Another theme is mentorship. The older brother usually imparts his knowledge of ninjutsu and life lessons to Naruto.
One common theme is self - discovery. The little brother OC often starts out not fully knowing his capabilities or place in the RWBY world, and throughout the story, he figures out who he is. Another theme is friendship. He forms strong bonds with the RWBY characters, like Ruby, who might become his best friend. And there's also the theme of adaptation. Since he's new to this world, he has to adapt to the rules, the Grimm, and the different cultures in Remnant.
Another theme is self - discovery. Sitka, through his interactions with the OC, might learn more about himself, his values, and his place in the world. The OC also might have their own journey of self - discovery. For example, in some fanfictions, the OC discovers their true purpose while helping Sitka on his quests.
One common theme is the discovery of new powers. Since it's supernatural, the OC often realizes they have some unique ability that helps them in the Until Dawn situation, like seeing the future or controlling the elements.
Family is also a big theme. Whether it's finding a new family with the Winchesters or reconnecting with their own lost family in the supernatural realm. In a lot of these fanfics, the OC kid forms a strong bond with Sam and Dean or other characters, and this family - like relationship becomes a central part of the story. It gives the story an emotional depth as they support each other through various challenges in the dangerous supernatural world.
Another theme is self - discovery. The god - oc is usually in a journey to figure out their true nature, purpose, and capabilities in the context of the supernatural world. It's like a coming - of - age story but in a more complex and otherworldly setting. For example, in many fanfics, the god - oc might start with only a faint idea of what they can do and gradually learn about their full potential through various experiences and encounters. This exploration of self not only includes their powers but also their moral and ethical stances in this new and often dangerous world.