They, like their neighboring planet's species, the Brekkians, possessed the natural ability to generate electrical discharges with their bodies, possibly due to the unusually strong magnetic field of their sun. It could have a mild effect of inducing a stasis in another humanoid, or it could kill.
In the 22nd century, a plague halted progress in the Ornaran society. It was found to be mitigated with the medication felicium from the planet Brekka. The originating plant could not grow on Ornara, forcing a continuing trade agreement between the two species. Unfortunately, felicium was later found to have powerfully addictive narcotic effects, with the result that all Ornarans had become addicted to the drug. The Brekkians exploited the situation, selling felicium to the Ornarans, while concealing from them the fact that the drug was no longer needed to control the plague.
Generations of drug addiction resulted in loss of intelligence and technical knowledge, and by 2364, the Ornarans no longer had the ability to maintain their interplanetary freighters that they needed to transport felicium from Brekka to Ornara. As a result, the freighter Sanction was destroyed above Brekka when a drive-coil malfunction made it impossible for the ship to maintain a stable orbit. Although the malfunction was fairly minor, the Ornarans' technical ignorance made the problem disastrous.
The Ornarans, along with the Brekkians, requested Federation assistance in repairing their remaining ships, but USS Enterprise-D Captain Jean-Luc Picard declined to render aid, citing Prime Directive considerations. This meant that the Ornarans would quickly start to withdraw from the drug, believing they were dying. (TNG: "Symbiosis")
"ore-NAHR-uns" was the pronunciation for this species' name from the script pronunciation guide. 
According to an internal reference document listing all the aliens seen in Star Trek: The Next Generation, which was circulated around the time of the production of TNG Season 5, the Ornarans were given the description of "Largely humanoid except for a natural electric charge, usable as a weapon, and a laterally-wrinkled lower nose with a u-shaped band of skin across the bridge."
According to the background book The Worlds of the Federation, in the aftermath of the events of "Symbiosis", the Ornarans began to suffer withdrawal symptoms but did not die, finally becoming aware of their addiction to felicium and the Brekkians' exploitation of it. However, in spite of this awareness, they chose not to break their dependence on felicium and willingly continued trading with the Brekkians for it.