Alpha Onias III was the third planet of the Alpha Onias system, in the Onias sector. This Class M planet was in or near Federation space, and was located near the Romulan Neutral Zone. The surface of this planet contained high, but non-toxic levels of volcanic gases, including sulfur dioxide, methane, and hydrogen sulfide.
Barash, a humanoid child, was hidden in a cavern by his mother after their homeworld was attacked in the mid-24th century. The cavern was equipped with neural scanners that were able to transform matter into any form imagined, so that Barash could live in safety. (TNG: "Future Imperfect")
According to the Star Trek: Star Charts, on page 61, the Alpha Onias system was located in or near to Romulan space, in the Beta Quadrant. Alpha Onias was a single star system. Primary was a Class M star with a magnitude of +9.