Omicron Ceti III was the inhabited third planet in the Omicron Ceti system of the Alpha Quadrant. Omicron Ceti III was suitable for colonization by humanoid lifeforms if the colonists were able to counter the harmful side effects of Berthold rays.
The planet was once considered the perfect planet for an agricultural colony, as its soil would grow anything planted in it. The climate was moderate, having a moderate amount of rain all year round. Unfortunately, this perfection was marred by the fact that the planet was bombarded by Berthold rays at such an intensity that any animal life could only survive for a week.
Multiple expeditions were made to colonize the planet, but all failed as the Berthold rays were not yet known to Federation science. The last attempt, the Omicron colony, led by Elias Sandoval, made planetfall in 2264. This group was able to survive because of the colonists' discovery of the pod plants and their mysterious spores, which protected the colonists from the Berthold rays.
On stardate 3417 in 2267, the USS Enterprise arrived at the planet, following the discovery of Berthold rays, to ascertain the exact fate of the colonists. Upon learning that the colonists were still alive, Captain James T. Kirk and Commander Spock were able to develop a method of freeing them from the influence of the spores. The colonists were evacuated to Starbase 27. (TOS: "This Side of Paradise")
In the 2340s, on stardate 22827.5, Lieutenant Anaanda Ziff was born on Omicron Ceti III. This lieutenant's personnel file appeared in a hallucination of Deanna Troi's. (TNG: "Eye of the Beholder" okudagram)
According to Star Trek: Star Charts (p. 37, "United Federation of Planets I", "United Federation of Planets II") and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), in 2378, Omicron Ceti III was a Federation member.
The Star Trek Encyclopedia (4th ed., vol. 2, p. 104) classified Omicron Ceti III as a class M planet. This reference work stated that of the 150 colonists who made the trip to this planet in the 2260s, approximately fifty of them were still alive when the Enterprise came to check up on them. This could be based on the fact that the starship's sensors detected one settlement on the planet's surface. Initially there were three settlements, with Sandoval's settlement having 45 individuals. Although it is implied in the episode that the other two settlements are active in 2267, this was never directly confirmed in the episode.