Omicron Theta was the home to an Earth colony, the Omicron Theta colony. By 2328, this planet was a destination on commercial transport routes. In the colony, Dr. Noonian Soong and his wife Juliana perfected their positronic brain and successfully created a series of Soong-type androids including Data and Lore. At least one archaeological dig was conducted in the mountains. The colony also contained farmland. (TNG: "Datalore", "Silicon Avatar", "Inheritance")
In 2338, after the planet was attacked by the Crystalline Entity, the cruiser USS Tripoli picked up a signal, which led the starship to this planet. The cruiser's crew reported finding a world with farmlands. A landing party traced the signal to a signal device on the surface. Near the signal device, the party discovered Data. Upon discovery, Data was activated by the device.
In 2364, the USS Enterprise-D visited the planet for a few hours. The ship's sensors found a Class M world with lifeless vegetation and devoid of insects and soil bacteria. An analysis by an away team from the starship ascertained that at the time the Tripoli discovered Data, everything on the planet was dead or dying. (TNG: "Datalore", "Rightful Heir")
Andrew Probert illustrated a handful of drawings to show how the set for Dr. Soong's lab might look. However, at least one of these concept artworks was labeled "Kiron III," the original name used for the planet. (Star Trek: The Magazine Volume 2, Issue 12, p. 49) (The planet's moniker was also Kiron III in the novelization of "Encounter at Farpoint"). In the script for "Datalore", the planet was identified as Data's Planet. Omicron Theta was the name of the system, not the planet.  This changed in "Silicon Avatar" when the planet, for the first time, was canonically identified as Omicron Theta. "ohm-ih-kron THAY-tah" was the pronunciation for this planet's name from the script pronunciation guide.