A nebula in the Gamma Quadrant that concealed the rogue planet on which the Founders of the Dominion lived in the Great Link until a Romulan and Cardassian attack in 2371.
The Omarion Nebula is where Odo finds his people. In The Search, at the start of 2371, he feels drawn to it and discovers a sunless planet with a lake of changelings, learning that his own kind are the Founders who rule the Dominion.
Later that year, in Improbable Cause and The Die Is Cast, Enabran Tain leads a secret fleet of the Romulan Tal Shiar and the Cardassian Obsidian Order to destroy the planet. The attack is a trap: the Founders have already left, and a huge Jem'Hadar force wipes out the fleet.
The Founders settle on another world, seen in Broken Link, and the nebula plays no further part in the series.