The Citadel of Oa is located on the planet Oa, and is the only inhabited area of the entire planet. It is the headquarters and meeting hall of the Guardians of the Universe. Millennia ago, the Guardians used the citadel laboratories to create a robotic army of soldiers to safeguard the universe. These beings became known as the Manhunters, but they later rebelled and left Oa permanently, taking up new refuge on the planet known as Biot. Centuries later, the Guardians established a new peacekeeping task force known as the Green Lantern Corps. The citadel became a training center and proving ground for the Corps. Members of the Green Lantern Honor Guard such as Kilowog and Tomar-Re recruited fresh warriors to train and serve in the Corps. The citadel houses the Central Power Battery of Oa - a reservoir of energy which embodies raw willpower. It is also where the mysterious Book of Oa is kept. The Guardians select chosen Green Lanterns to serve as Keepers of the book. In the first wave of the Green Lantern Corps, the Keeper of the Book of Oa was Tomar-Re. With the rebirth of the Corps, the position was handed to the ever-stern Slyggian, Salakk. The Oan Citadel also houses hundreds upon hundreds of Sciencells - spherical containment capsules where the worst intergalactic villains of the universe are imprisoned.
The Oan Citadel has been destroyed and rebuilt numerous times. It was first destroyed during the Crises on Infinite Earths in an attack which resulted in the deaths of several guardians. It was rebuilt shortly thereafter, but abandoned when the Guardians sent themselves into exile to be with their feminine counterparts the Zamarons. The Citadel was destroyed again years later when Hal Jordan, infected by the entity of fear known as Parralax, entered the Central Power Battery and absorbed all of its power. When the Green Lantern known as Kyle Rayner possessed the power of Ion, he resurrected the Guardians and they established a new Green Lantern Corps, as well as a new, larger citadel and several sector houses.