About three billion years ago, the Oans created the Green Lantern Corps as their new peace-keepers, agents from across the universe empowered by a central power battery on Oa. A small group of Oans felt these methods too weak to be effective, and left Oa to follow their own path. This splinter group would one day evolve into the race of Controllers. About two and a half billion years ago, all women left the planet of Oa. These women became Zamarons, dedicated to preserving their physical forms through combat, even as the male Oans allowed their forms to gradually atrophy into the dwarfish ones of the present time. Both sexes, though reduced in number, felt no need to produce new offspring, since they were themselves immortal. As the Green Lantern Corps grew in influence and universe-wide power, the 36 remaining Oans devoted themselves to providing them with the power and wisdom needed in battling evil. 

(JLA #84, 86, 97 (fb)) - <20,000 years ago> The Guardians observed the Burning, the dominant lifeform on Mars that was extremely savage and chaotic. To control them they placed mental blocks on the Burning, giving them an innate fear of fire, and transforming them into the more peaceful race of Martians. One transport ship bringing Burning to Oa for genetic reprogramming crash landed on Earth, killing the Guardians aboard. Only one injured Burning survived, but he was killed by Vandal Savage.

(Green Lantern: Emerald Dawn #4 (fb)) - The Guardians recognized the Tchk-Tchkll, a warfaring race, as a threat to the cosmos, so they sent the Green Lantern Corps to drive the Tchk-Tchkll back to their home planet, then had the Corps erect an energy shield around their planet, derailing any future plans of conquest. The Tchk-Tchkll hated the Guardians for this and created the gestalt entity Legion to destroy them.

(Martian Manhunter II #21) - The Guardians sentenced Tybalt Bak’sar, who'd been captured by Abin Sur,  to a lifetime of confinement in the prison planet for his destruction of the planet Zapher. Tybalt pleaded that he’d destroyed the planet because they’d imprisoned his people, and vowed to destroy the Guardians’ Green Lantern Corps.

(Adventures of Superman #436(fb)) - <30 years ago> The Guardians sent the Corps to prevent the Manhunters from intercepting a Kryptonian birthing matrix containing Kal-El that was heading towards Earth. The acquisition of the last Kryptonian would tip the balance between the Manhunters conflict with the Guardians, and the Corps fought valiantly. They were taken down by the Manhunters, but prevented them from getting the matrix.

(Green Lantern: Emerald Dawn #5, 6) - Oa; The Guardians put themselves into suspended animation for their yearly meditations, but were interrupted by Legion, who invaded their inner sanctum. The Green Lantern Corps faltered against Legion, and the Guardians, fearing that their demise would compromise order in the universe, planned to abandon Oa. Rookie Green Lantern Hal Jordan told them to belay their plans, and single-handedly defeated Legion. Jordan questioned the Guardian's past decisions, implying that the way they handled the Tchk-Tchkll only created Legion, a new cosmic threat. The Guardians were taken aback, but respected Hal for both his courage to question them, and the potential he displayed in defeating Legion.

(Green Lantern: Emerald Dawn 2 #1) - The Guardians determined that Lantern Hal Jordan was still not fully trained and somewhat unpredictable, so they assigned Sinestro, a model of an orderly Lantern, to be Hal's mentor on Earth.

(Green Lantern: Emerald Dawn 2 #4, 6) - The Guardians learned that Sinestro was running his home planet as a tyrant. He was the first Lantern to become corrupted, and the Guardians determined that duty to the Corps itself could corrupt. They sent the Fists of the Guardians to Earth to capture Sinestro and bring him to Oa for a trial. The Guardians found Sinestro guilty of abusing his power and banished him to Qward. At Tomar-Re's suggestion they appointed Katma-Tui, who led a revolt against Sinestro's rule, the new Green Lantern of Korugar.

(Green Lantern III #100) - The Guardians were held hostage by Sinestro, but freed by Green Lantern Hal Jordan and time-traveling Green Lantern Kyle Rayner. The Guardians were displeased with the idea of a future Lantern in their own time, and demanded Kyle allow them to send him to his own time. Hal and Kyle convinced them to let them defeat Sinestro first, and after Sinestro was captured the Guardians tapped the central power battery to send Kyle 10 years into the future. Unfortunately Sinestro, though defeated, managed to force Hal along for the ride.

(Green Lantern II #126, 127) - The Weaponers of Qward invaded Oa, incapacitated the Guardians and yoked them to the Central Power Battery, intending to use the Battery's power and the Guardian's immortality to create the ultimate weapon. Hal arrived on Oa, but was trapped in the Battery himself. The Guardians managed to call in the Corps with Hal's help, but the Weaponers erected a yellow vortex that trapped most of the Lanterns. One dying Lantern made his way to Oa and freed Hal Jordan. Hal to deactivate the vortex and free the Corps. The Corps utterly defeated the Qwardians, but a number of Lanterns perished in the battle. The Guardians acknowledged that there could never be true victory in war.

During the Crisis, caused as a direct result of Krona’s never forgotten experiment, they split into factions, battling even each other, and making mistakes in which countless lives were lost and 14 Oans destroyed. The remaining Oans gave up their guardianship, leaving the Green Lantern Corps with all former powers, but without direct control.

(Green Lantern III #7, 8) - When the Old Timer stole towns from across the universe to create a Mosaic World on Oa, Hal Jordan used Oa's central power battery to call the Guardians out of their self-imposed exile to stop the Old Timer. The Guardians were losing their battle against the Old Timer until Hal intervened. He gave them advice on fighting, then had John Stewart weaken Old Timer's resolve using their mental link. The fight ended with the Guardians taking Old Timer's life. The Guardians told Hal their time away gave them time to reflect on their past mistakes and they intended to resume their duty protecting the universe with a newfound humility. The Zamarons would return to Oa after they gave birth to the Guardian's children. The Guardians also announced they would reform the Green Lantern Corps, and told Hal they would call him back after they decided how to best use Hal, John Stewart and Guy Gardner to restart the Corps.

(Green Lantern III #14, 16, 18) - John Stewart approached the Guardians and informed them about the Horde, a hostile alien race attacking the human communities on Mosaic World. He asked the Guardians to remove the aliens from the Mosaic. The Guardians refused, they wanted to see how John would deal with the situation. More communities started fighting, so John erected giant barriers separating all the communities on the Mosaic. The Guardians remembered the mistake they made dealing with the expansionist Tchk-Tchkll. Separating them from other races led to their suffering and plans of revenge, and the Guardians couldn't help but wonder if the Mosaic communities wouldn't follow the same path. The communities tore down John's barriers and started warring against each other, but John, Hal Jordan, Brik and Chaselon temporarily put down the revolt. John confronted the Guardians with his suspicions that the Guardians had recharged the central power battery and could send the Mosaic communities back to their homeworlds at anytime. The Guardians admitted he was right, and told him the Mosaic served a purpose as an experiment in inter-species relations. They would continue the experiment with or without John, so they recommended he stay around as the Mosaic's protector.

(Green Lantern III #33-35) - Hal Jordan used his power ring to contact the Guardians, and told him that Entropy had abducted the New Guardians and swore to destroy the GLC because the Guardians of the Universe had "broken the third law." The Guardians told Hal they saw no need to interfere, and refused to discuss Entropy's cryptic statement. Hal went to Oa and demanded answers. He also chastised the Guardians for cutting off contact with the New Guardians and forgetting about them. The Guardians told Hal their plan for order in the cosmos was eons in the making and not subject to scrutiny by him. They also revealed they lied when they told him they'd learned humility during their self-imposed exile, but said the lie was necessary to get Hal to help reform the Corps. Entrophy and his army attacked the Guardians. The "third law" they'd referred to was the third law of thermodynamics, that everything descends into chaos, and Entropy saw the Guardians as going against nature by trying to preserve order. The Guardians foresaw Entropy's attack, and had the GLC on hand to fight him. Entropy made the GLC question their loyalty to the Guardians, so Ganthet decided to explain the end-result of the Guardians' work. Krona's terrible experiment ages ago used up a billion years of the universe's usable energy. Because of this, when the current universe collapsed it would not be able to create a new singularity, and from there a new universe. By creating a universe of order the Guardians hoped to preserve enough usable energy to one day ensure that a new universe would emerge from the old. They had to abandon the shattered GLC, then return to guide them to start a new age of order, the "Epoch of Dynamic Guidance." The GLC regained their motivation and defeated Entropy, and the Guardians gave the GLC their new task, each member must go out and recruit twelve new members.

(Green Lantern: Mosaic #10) - John Stewart petitioned the Guardians to send the cities of the Mosaic World back to their home planets. The Guardians told John they needed to observe the progress of Mosaic, because as worlds gained the means of space travel more alien cultures across the universe would start interacting, and they hoped the Mosaic would give them an idea of how different species would get along. John showed them all the progress Mosaic had made, including examples of interspecies cooperation and the planning of a fair that promoted cultural exchange. The Guardians were so pleased with John’s work they denied his petition and demanded he keep working to bring the cities of the Mosaic closer together.

(Green Lantern: Mosaic #16, 17) - When a number of superheroes from Earth came to Mosaic World to return the Mosaic cities back to their home planets the Guardians appeared. They told the heroes they would not interfere; they were interested in how the situation would play out. In the end most of the residents of Mosaic opted to stay where they were.

(Green Lantern: Mosaic #18) - The Guardians gave a portion of their power to John Stewart and told him he was a prophesized savior of the universe. They revealed that the only reason they made multiple Earthlings Green Lanterns and allowed the Green Lantern Corps to temporarily move to Earth was to maneuver John into the position of savior of Mosaic.