Originally, the Onderonians were primitive humans who inhabited areas close to the jungles of Onderon. They constructed fortified settlements as a means of defense against the dangerous predators of the jungle. These settlements eventually grew into massive walled cities, such as Iziz. Some outcasts chose to live in the planet's jungles, where they tamed and rode creatures known as rupings.
During the times of the Separatist Crisis and the Clone Wars, Onderonians had representation in both the Galactic Republic's Galactic Senate and the Confederacy of Independent Systems' Parliament by Mina Bonteri. Ramsis Dendup, the Onderonian king, desired neutrality for their world during the Clone Wars, believing both the Republic and the Confederacy to be corrupt.
At the onset of the Clone Wars, the Separatists ended Dendup's rule and appointed Sanjay Rash as the new king of Onderon. Furthermore, Separatist agents assassinated Bonteri, even though she had become a full supporter of the Separatist cause, following the Confederate–Republic peace initiative. A group of Onderonians, led by Lux Bonteri, Steela Gerrera, and her brother Saw Gerrera, sought assistance from the Jedi Council to reclaim their planet and reinstate Dendup as their rightful king. With the Jedi's help, the Rebels successfully forced the Separatist Droid Army under General Kalani to withdraw from Onderon.
After the Republic was restructured into the Galactic Empire, Gerrera, his followers, and numerous civilian refugees were labeled as insurgents by Admiral Wilhuff Tarkin. They sought refuge in the jungles to avoid the Imperial forces. Clone Force 99 was dispatched to deal with Gerra's insurgents, but they refused to carry out the order. Later, a new Imperial Elite Squad was deployed, eliminating several of Gerrera's fighters and killing numerous civilian refugees. Gerrera initiated a bloodly insurgency against the Empire on Onderon, but ultimately deemed his homeworld a lost cause and escaped.