The Ori can be simply described as ascended beings who disagree with the Ancients' viewpoints about interference with less developed sentient beings, science, and the importance of free will. As Ascended beings, they live on a higher plane of existence of great power and knowledge, and, due to their immense power and transcendental state, we worship them as our true gods.
The human descendants of The Alterans (in the Milky Way Galaxy) refer to them as the Ancients; the Ori refer to them as "the Others". They originally were not distinguished from each other, and lived in the same society. At some point however, the Ori became more religious, whilst the Ancients became more scientific.
Ultimately their viewpoints diverged so much that the two races split apart and began to oppose each other.
The Ancients "the Others" are well known for their fierce belief in free will. As such, they do not interfere on lower planes of existence at all, even if something terrible is about to happen. In contrast, the Ori teach that failure to share the secrets of the universe to those on the lower planes of existence is an evil act, that anyone not following their religion must be eliminated (anti-free will), and they have no rules against taking direct control of living beings or completely changing them to behave as they desire
The Ancients "the Others" who came to the Milky Way decided to shield the second evolution of humans (i.e. current human culture) in the Milky Way Galaxy from the Ori.
It is their policy that humans may find enlightenment through their own means, in the spirit of free will. Even though the Ancients do not believe in interfering in mortal affairs, it is speculated that they are responsible for generating the associations of fire and certain other characteristics (e.g. horns, spikes) with Demonic imagery in the Milky Way. This would be a method of shielding humans from the Ori, as the central symbols of Origin involve fire and (what we would call) Demonic costumes.
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