Both New Republic and Imperial Star Destroyers fight against the Dominion during the war.
The Otiosian War, also known as the Otiosian Conflict and the Otiosian Crisis, was a two-year galactic conflict fought between the forces of the New Republic and the Imperial Remnant, and the Dominion, between the years of 21 ABY and 23 ABY. The conflict resulted from the long-standing dispute over the possesion of parts of the Otiosian sector, which was claimed by both the New Republic and the Dominion, although the Imperial Remnant had settled much of the area. It was one of the most destructive and bloody wars in galactic history. By the time the conflict ended, there was a catastrophic death toll for all powers involved, and the war left the two powers ill-prepared to fight the Yuuzhan Vong invaders two years later.