Omega IV was a planet and homeworld for the Omega IV natives, a pre-warp humanoid species. This species was divided into two ethnic groups, the Kohms and the Yangs, who had fought a biological war centuries ago. (TOS: "The Omega Glory")
In 2293, the location of Omega IV in the Milky Way Galaxy was labeled in a star chart that was in Captain James T. Kirk's quarters aboard the USS Enterprise-A. (Star Trek VI: The Undiscovered Country, okudagram)
While the planet had different land masses from Earth, Omega IV shared a history nearly identical to Earth's, up to the 1990s or slightly beyond. Omega IV once had a nation called "The United States" similar to Earth's United States of America, with the same handwritten Constitution of the United States, the Pledge of Allegiance and United States flag identical to that of Earth's.
The stand-off on Omega IV between the two superpower had escalated into open biological warfare. One of the primary weapons of the war was a virus that disrupted the body at the molecular level, separating water molecules from other organic compounds until there was nothing of the body left but dehydrated crystals. The virus was still present on the planet in the 23rd century, though only a brief exposure to the atmosphere at that time was needed to provide a lasting immunity to the virus. The Asiatic Kohms, originally called "Communists", won the war against the Caucasian Yangs originally called "Yankees". As the survivors of the war adapted to the toxic environment, they acquired a naturally long life span. They could live to be more than a thousand years old.
The Yangs were driven from their cities, and took to the ways of the native Americans for survival. By the 23rd century, civilization on Omega IV had returned to the iron-age level. The Yangs, dressed in tribal attire, lived in the hills and mountains surrounding the ruined cities. The cities were inhabited by the Kohms, who dressed similarly to Earth's Mongolians. The Yangs were attempting to take their old cities back from the Kohms.
In 2268, Captain Ronald Tracey of the starship USS Exeter beamed down with a landing party to survey the planet for research purposes. He found the Kohm city and established friendly relations with the Kohms. However, when some of the landing party beamed back to the ship, they carried the virus back with them. Within a short time the entire crew of the Exeter was killed.
Driven insane by the death of his entire crew and the loss of his ship, Captain Tracey sided with the Kohm villagers in their conflict with the Yangs. He used his phaser to drive off a Yang attack in violation of the Prime Directive. When the USS Enterprise arrived to investigate, Captain Kirk refused to help Tracey in the battle against the Yangs. He took Tracey away from Omega IV and allowed its people to rediscover their own "history and liberty." (TOS: "The Omega Glory")
In a deleted scene from the episode, the landing party speculated that the Yangs and Kohms might, in fact, be Human.
According to the Star Trek: Department of Temporal Investigations novel Forgotten History, the American paraphernalia on Omega IV were not indigenous documents, but in fact replicas left behind by the crew of ECS Philadelphia (β) in the late-2140s, to inspire the native Yangs in the struggle for freedom against the Kohms.
According to the novel Forged in Fire, a key component in the virus used by The Albino to murder the sons of Kang, Kor, and Koloth came from Omega IV. The Albino also infected Sulu with the virus, but Sulu was immune to it because he had visited Omega IV.