The Federation colony where Noonien Soong built Data and Lore, wiped out by the Crystalline Entity in 2338.
Omicron Theta was a Federation colony world where Dr. Noonien Soong lived and worked in secret, with his wife Juliana. There he built the androids Lore and Data. Lore proved unstable and dangerous, and the colonists asked that he be shut down. Soong then built Data, who was loaded with the colonists' memories and journals.
In 2338 the Crystalline Entity attacked the colony and stripped it of all life. Data was found by the crew of the USS Tripoli, leading to his joining Starfleet. In 2364 the Enterprise-D visited the dead world, found Soong's laboratory and Lore, and reassembled him, beginning Lore's long and destructive history. Lore was later revealed to have helped the Entity attack the colony.
Omicron Theta is the starting point for one of the Next Generation's most important stories. It explains Data's origins and his bond with Lore and Soong, and it shaped Data's quest to understand humanity and his own nature.