From The Stargate Omnipedia
A race of ascended entities who gained energy through the worship of mortal beings.
The Ori were once the same civilization as the Alterans, but (for want of a better term) believed in religion, where their brothers relied more heavily on science. A gap was eventually bridged between the cultures. In consequence the other Alterans left for the Milky Way Galaxy(to later be known as the Ancients).
The Ori used their ascension and knowledge of the universe as justification for praise and worship. They were still aware of the Ancients, and shunned them, for they abandoned the "path," and believed in the opposite philosophy. Not sharing the secrets of the universe to those on the lower planes of existence is an evil act and those who practice it must be eliminated.
The Ori conceived the Origin faith, one that placed them in the center of creation, and wrote down their guidelines in the Book of Origin to be administered to followers through Priors, missionaries and teachers of Origin.
Due to their numbers, the Ori possessed an equal level of power to the Ancients on the upper planes of existence. Dr. Daniel Jackson suspected that it was because of this deadlock in strength that the Ancients were able to prevent the Ori from the knowledge that the Milky Way galaxy is populated. But the Ancients will not stop free will, nor will they prevent unbelievers from converting.
The Ori used the Doci as their "mouthpiece" for communicating with lesser beings. They were to possess his body to spread their demands and will. During possession they could be identified with fiery eyes.
The entire purpose of Origin was a lie, as the Ori did not actually offer enlightenment. This was just a pretense to ensure the worship of lesser beings. This created a transfer of energy from the worshiper to the Ori. Only by having beings believe and worship in them could the Ori thrive, feeding off this energy field. When a believer died they were not taken to live among their gods, but instead unceremoniously winked out of existence. The Priors were not made aware of this truth, and most were so programmed in Origin that they were hopelessly lost.
The discovery of Merlin's weapon, a piece of technology capable of destroying ascended beings, proved to be the tide-turner for the struggle against the Ori. Using Merlin's knowledge Daniel Jackson rebuilt the device and sent it through to the Ori galaxy without interference from the Ancients. Once there the Ori themselves were annihilated.
This did not stop the human followers, however, and SG-1 traveled through the Ori supergate to Celestis to locate the Ark of Truth, and successfully brainwashed the Doci, his Priors and the followers under them of the Ori's true intentions.
HOMEWORLD - Celestis
FIRST APPEARED - Origin
Avalon, Part 2 - An Ori Prior restores Vala's life force after her death on an Ara and instructs she and Daniel to follow him to the Plains of Celestis for an audience with the Ori.
Origin - Daniel encounters the Ori and realizes that, because of he and Vala's visit, they are now aware of the Milky Way.
Beachhead - The Ori make their first attempt to enter the Milky Way by seizing planet Kallana and transforming it into an artificial singularity.
The Fourth Horseman, Part 1 - The Ancient Orlin descends upon Earth to construct an antidote for a Prior plague, while also explaining that the Ori require worshippers to survive.
The Fourth Horseman, Part 2 - Orlin reveals the Ori's true interest in the Milky Way -- they are on their way to destroy the Ancients.
The Shroud - Daniel Jackson and SG-1 activate Merlin's anti-Ori weapon and send it through the Supergate, wiping out the ascended beings.
Dominion - Adria claims that Merlin's weapon didn't kill the Ori, but a Baal symbiote claims after it takes her over that they were indeed destroyed.
Stargate: The Ark of Truth - Her powerbase compromised, Adria -- the last of the Ori -- is engaged in eternal combat by Morgan Le Fay. The Ori threat is over.