The Onderonians were originally primitive humans who lived near the jungles of Onderon. To protect themselves, they built fortified settlements to keep the jungle's dangerous predators at bay. Eventually these settlements would become enormous walled cities such Iziz. Some outcasts ended up living within the world's jungles, where they domesticated and rode rupings.
During the Separatist Crisis and the Clone Wars, the Onderonians were represented in both the Galactic Republic's Galactic Senate and the Confederacy of Independent Systems' Parliament by Mina Bonteri. However, the Onderonians' king, Ramsis Dendup wished for their homeworld to be neutral during the Clone Wars since both the Republic and the Confederacy were corrupt.
At the onset of the Clone Wars, the Separatists put an end to Dendup's monarchy and installed Sanjay Rash as the new Onderonian king. Additionally, Bonteri, despite having fully joined the Separatist cause, was killed by Separatist agents after the Confederate–Republic peace initiative. A group of Onderonians under the leader of Lux Bonteri, Steela Gerrera and her brother Saw Gerrera asked for help from the Jedi Council to help them reclaim their home and reinstall Dendup as their true king. With help from the Jedi, the Rebels were able to force the Separatist Droid Army under General Kalani to retreat from Onderon.
Following the reorganization of the Republic into the Galactic Empire Gerrera, his group and several civilian refugees were branded as insurgents by Admiral Wilhuff Tarkin. They hid in the jungles to evade the Imperial forces. He sent in Clone Force 99 to deal with Gerra's insurgents, but they refused. Later, a new Imperial Elite Squad was sent in, eliminated several of Gerrera's fighters, and killed several of the civilian refugees. Gerrera launched a bloodly insurgency against the Empire on Onderon but ultimately declared his homeworld a lost cause and fled.