OU1, also known as "Draterra", is a world that was once ruled by dragons. In the late BC era, they were wiped out, but in their death Dragonbloods were created. These are part human, part dragon shapeshifters who's dragon side grows over generations. Unfortunately, humans have become such a prominent species on Draterra that even Dragonbloods struggle to survive there.
Just like OU1, here too dragons appear to have died out. However, some dragonbloods have travelled here to live, as this world currently has nothing against them - though it doesn't know of them yet.