I'm not sure comparing OC-centered fics with particular ships is the best analogy. I mean, you can safely assume that all R/Hr or R/S fics are going to feature two very popular characters and a popular plot element (romance), and that they'll have certain other canon characters in supporting roles and be set in a particular time frame -- while about the only thing OC-centric fics have in common is the fact that they don't have a canon character in the lead. Otherwise, they're all over the map. Somebody who's very interested in, say, a seventeenth-century historical fic about the origins of the Statute of Secrecy might not be interested in a Beauxbatons fic set during Harry's sixth year, and vice versa.