My main fandom is the UK TV show Primeval and the space with all the OCs is primeval_denial. There was a point in time when the OCs were a fairly bitter point of contention in the LJ fandom. There was wank, though considering how bad fandom wank can get it was mostly pretty mild. At the end of the day Denial turned out to be the comm that had sticking power after the cancellation of the show and the "no OCs" comms are dead. All the active participants at denial now either like, or at least tolerate, the OCs.

They happened really, because the military characters in the show were popular in the fandom so people wanted to have a proper unit of military characters for them to interact with and the show simply didn't provide them. They just always showed up with a team of nameless extras wearing black. I think what happened next was more or less a variant on "don't invent your own OC if there is already a suitable character in the show" - basically "don't invent your own team of support soldiers if the fandom already has some and doesn't mind sharing".