Oshionian Prime was the prime planet in its star system. This planet was known for its scenic marshlands.

In 2371, Neelix had reserved the holodeck for himself and Kes so that the two could take a moonlit stroll through the planet's aforementioned marshlands. (VOY: "Fury")

This planet was only mentioned in dialogue.
The Star Trek Encyclopedia (4th ed., vol. 2, p. 115) located Oshionian Prime in the Delta Quadrant.