Oceanus IV
Oceanus IV was the inhabited fourth planet in its star system. This planet was located somewhere in the Alpha or Beta Quadrant.
In 2368, Starfleet added a diplomatic mission to Oceanus IV to the mission roster for the USS Enterprise-D. This addition cut down the time to explore the Phoenix Cluster from five weeks down to two. (TNG: "The Game")
This planet was only mentioned in dialogue.
In the Star Trek: Star Charts ("United Federation of Planets I"), Oceanus IV was listed as a Federation member in 2378.
The Star Trek Encyclopedia, 4th ed., vol. 2, p. 99 identified Oceanus IV as a Federation planet.