The race's animosity with the Goa'uld began millennia ago, when the Goa'uld Belus discovered and invaded their homeworld of Oannes. Belus drove the Ohnes from their ocean-born cities by bombarding the seas from orbit while the System Lord's slavers waited on the surface to capture the fleeing inhabitants who were transported as slaves to Belus's ship in orbit. Confident in his victory, Belus left only a token force behind, intent on returning to Goa'uld territory with his new prize captives.
His ship was barely one day away when the Ohnes overwhelmed their captors and moved to seize control. Rather than be defeated by mere slaves, Belus decided to flee his command ship with his First Prime while setting the self-destruct on his vessel which slaughtered thousands of Ohnes that were on board. At the same time, on the Ohne homeworld, the Ohnes launched a counterattack against the Goa'uld and Jaffa that remained on their planet. Without their 'god', the invaders were quickly routed and the Ohnes retook their world.
Interrogation of the prisoners failed to obtain the location of Belus' homeworld so the Ohnes used their memory retrieval to obtain what information they could which killed the captives in the process.
In addition to breathing in the ocean, they can comfortably breathe on land, though it is possible they are more dependent on water breathing than air breathing. They have shown themselves to oppose the Goa'uld. One of their own, Omoroca, went to Earth to help foster a rebellion against Ra, though she was ultimately killed by Belus. Her mate, Nem, spent four thousand years alone before he discovered her fate. (SG1: "Fire and Water")
This race has a lifespan that lasts 5,000 years and their young reach maturity by the age of 200. Their society is divided into castes; the two primary ones being the Warrior caste and the Scholar caste though several lower castes do exist.
This race seem to have near-ascended abilities as Nem was able to render SG-1 unconscious by firing a blast of energy at them from his hand.