OCs are more then OK to. Actually a single OC would be realistic. For now it has been confirmend that, at least to some extend, the more known an idea/setting/and so on is the large part of its world would appear on Earth after The Arrival. So a single character born and kept only in a single mind would easily "Arrive" to Earth, but a house of that character would have seriously low chances of doing so. Note that for each characters the rules of its world apply, not those of Earth, as for when two "Arrived" character clash can you guess, out of the two characters, whose world's rules would apply?