A believable OC in Phantom of the Opera fanfiction can be created by making them interact realistically with the existing characters. For instance, if your OC is a new soprano, how would Carlotta react to her? Would she be jealous? Also, give the OC a distinct voice, both literally and figuratively. Their way of speaking, their accent if they are from a different place, should be unique. Moreover, think about how the Phantom would view the OC. What would draw him to the OC or make him despise them? By considering all these elements, you can make an OC that feels like it belongs in the world of the Phantom of the Opera.