(covers information from several alternate timelines)
Omicron Ceti was the primary of the inhabited Omicron Ceti star system. There was an inhabited planet, Omicron Ceti III, in this system. After a failed attempt at colonizing this planet in the 2260s, the Federation would successfully attempt again and, by the mid-24th century, the colony was thriving. (TOS: "This Side of Paradise"; TNG: "Eye of the Beholder")
In 2366, in an alternate timeline, during the Federation-Klingon War, the location of Omicron Ceti was labeled on a tactical situation monitor in the ready room aboard the USS Enterprise-D (TNG: "Yesterday's Enterprise", okudagram)
Background information Edit
In the real world Omicron Ceti is also known as Mira. In the Star Trek universe however, the name Mira is used as an alternative shorter name for the red giant star Mira Antlia of the Mira Antliae system visited by the Enterprise-D in TNG: "Conspiracy". According to Star Trek: Star Charts (p. 37), Mira / Omicron Ceti was a Federation system located in the Alpha Quadrant in the late 24th century.
A lost Federation colony on the fourth planet, Omicron Ceti IV, was seen in the comic book story Old Loyalties.