O
O is a planet four light years from Hain, described in the title-story of the collection A Fisherman of the Inland Sea. Its people are known as ki'O and it is notable for its unusual four-person "sedoretu" marriage system (a set combination of both genders and sexual orientations). Two more tales about this world and its customs are found in the collection The Birthday of the World.
Read more about this topic: Terra (Hainish Cycle)