First, give your OC a clear backstory. For example, if they're a vampire, explain how they were turned and what their life was like before. Second, give them unique traits. It could be a physical feature or a personality quirk like being overly superstitious. Third, make sure their motives fit the world of 'The Originals'. If they're in New Orleans, they should have a reason to be there.
Start with their origin. Are they from a long - lost vampire clan? Or perhaps they're a witch with a unique heritage. This will set the foundation for who they are. Then, look at their skills and abilities. If they're a fighter, how did they learn? What kind of fighting style do they have? In terms of personality, make it multi - dimensional. They can be brave but also have moments of fear. They should be able to adapt to the dangerous and often unpredictable world of 'The Originals'. For example, if they meet Klaus for the first time, their reaction should be both cautious and curious, as he is a very complex and dangerous character.