Onderon was mostly a wild planet with thick jungles in ancient times. There were originally two main cultures on Onderon; one lived in the wild in small, hidden villages, while the other lived in a large, fortified citadel they called Iziz. The planetary government was situated in the citadel and sometimes spoke only for its inhabitants and not the entire planet's population.

During these times, a split emerged and more foreign races moved in. Mandalorians established themselves on Dxun, the planet's closest moon, and the planetary government became engaged in a civil war on Onderon. Iziz became embroiled in civil war until the Jedi aided the capitalists to re-establish the royal line. However, there is no indication that the Mandalorian group established on Dxun ever moved away and the planet has always been associated with Mandalorian history. Meanwhile, on Onderon, civilization continued as normal.

Near the end of this planetary conflict, when those two factions were still at war, there was a powerful dark Jedi spirit who plotted to bring back the powers of the Sith by corrupting the ruler of the citadel and making her his apprentice. His plan, however, was foiled by the ancient Jedi Order and peace was more or less restored to the planet. The citadel has some elaborate catacombs that worm their way underground for hundreds of meters, and most of the important people of the past are buried inside. It is said that even the fallen queen, the Sith's apprentice, was buried somewhere in there in the distant past.

Even though the inhabitants tend to bicker among themselves sometimes, Onderon is a nice planet to live on. Onderon is the planet with the highest population in this rich planetary system. Richer in history and resources than the other planets, Onderon is the most tactically important planet in this star system and thus is currently the seat of government for the system.

Onderon is orbited by a lush green moon, Dxun. In times past, Dxun was used as an ancient Mandalorian base, attesting to the Mandalorian influence seen throughout Onderon. This influence can most easily be seen in the capital city of Iziz, where many citizens of Mandalorian descent still reside.

In recent days, the system was developed extensively and as such today Onderon rests in a similar state to Coruscant. The jungles and forests are tamed, residing in controlled sections of the cities. A vibrant material extraction and production business has flourished, bringing in the wealth that caused over 10 billion credits of development to begin. Today, Onderon is the home of the Inner Rim rich and famous and has an abounding population. Its shields are arguably as strong as the famous Coruscanti defense shields and the planet is protected by a series of defensive satellites and space stations. The products of this system bring in traders from throughout the galaxy to buy and sell with the populace and the corporations based here.