- The Green Lantern Corps: Oa is the headquarters of the Corps, but during the Parallax war, many of the Corps' members were killed and the planet was also severely damaged and as a result it has been almost deserted.
- Guardians of the Universe: The Guardians were the original inhabitants of the planet and they were the ones, who made it the Corps' home but after the Parallax war the Guardians have all being killed.
- The Manhunters: The Manhunters were created by the Guardians and were the Corps' predecessor but after they left Oa and the Guardians after they were re-purposed by Parallax as weapons of fear.
In the Comics
One of the oldest planets in the universe, Oa serves as the home and headquarters for a race of blue-skinned powerful humanoids who have dubbed themselves the Guardians of the Universe. The Guardians administer the Green Lantern Corps, a group of powerful universal police officers equipped by the Guardians with green-colored power rings along with green lanterns with which to charge the rings. According to the story Heart of a Star in the Sandman: Endless Nights graphic novel, Oa's star is called Sto-Oa (the Light of Oa) by the children of the planet's inhabitants. It was formerly the center of the universe until the events of Infinite Crisis. Oa appears as a desert-like, lifeless planet. The main feature is the Guardians' city.
Green Lantern Corps member Hal Jordan, under the influence of the evil fear entity Parallax, kills all but one of the Guardians and most of the Corps before draining Oa's Central Power Battery of its energy. During the Circle of Fire event, what remains of the planet served as a base of operations for the villain Oblivion.
The planet is reformed sometime later by Jordan's old friend and associate Tom Kalmaku in the graphic novel Legacy: The Last Will and Testament of Hal Jordan. After attacks by Superboy-Prime and the Spider Guild, the Guardians fortify Oa by creating a planetwide armor and defensive system to prevent attacks. The full capability of the new systems is still unknown.
After Sinestro killed the Guardians for their Third Army plot, Hal Jordan became the new leader of the Green Lantern Corps, the planet was attacked and destroyed by Relic. Oa lies mostly in ruins, the power battery cracked, and the rings of the Corps lie in a pile, unable to locate new bearers due to the apparent death of Mogo. Statues of legendary Green Lantern Corps members line the hall where the Guardians formerly met.