The Omicron system was an inhabited system. It had at least four planets. The fourth planet, Omicron IV, was the homeworld for the Omicron IV natives. Crew members aboard Federation starships used the primary Omicron as a reliable point when computing distances between astronomical locations.
Lieutenant Commander Data often told an "amusing" anecdote concerning how he computed the distance between the Omicron system and the Crab Nebula. (TOS: "Assignment: Earth", TNG: "Manhunt")