factually incorrect. i've come across a bunch of people that make ocs from the same worlds together. there was a game way back in the lj days that had people need to make ocs from cards. i've also seen what i assume are friend groups throwing ocs on memes that they clearly made together, so while the genre 'oc' doesn't have castmates as a general rule, it's not true that some ocs might not have some cast from the same oc pool if a friend/rp partner makes a sister/lover/enemy for them.