The Deepglow Oarfish is a Legendary fish. making it one of the rarer catches available through fishing.

Obtainment:

The Deepglow Oarfish is obtained by fishing at the Cursed Ship while using Abyssal Bait.

Additional Information:
The Cursed Ship fishing spot is commonly accessed by glitching outside the ship, as the normal layout does not provide a standard place to fish. Many players use this method to reach the required location.

A safer alternative is to sail from Dock 2 toward the large "+" shaped structure on the Second Sea horizon. The platform on the back of the structure can be used as a valid fishing spot, allowing you to catch the Deepglow Oarfish without using any glitches.