The oarfish is one of the world's longest bony fish, a group that includes almost all fish except sharks and rays. Whale sharks are the largest fish in the ocean. Oarfish have been measured to reach up to 26 feet (8 meters) in length, and can grow to be nearly 50 feet (15 meters) long. Live oarfish are rarely seen as they mostly live deep in tropical oceans. They are known to swim to a depth of 360 feet (110 m) beneath the surface, using its long dorsal fin to precisely control its movements. The fish swim with their head upright and their tail hanging beneath them, and can easily move backward and forward and up and down quickly.

The oarfish have tapering, ribbony silver bodies with a pinkish to cardinal red dorsal fin. The dorsal fin originates from above their small eyes and runs the entire body length. Of the approximately 400 dorsal fin rays, the first 10 to 12 are elongated to varying degrees, forming a trailing crest decorated with reddish spots and flaps of skin at the ray tips. The pelvic fins are similarly elongated with 1 to 5 rays each. The pectoral fins are greatly reduced and situated low on the body. The rare fin is completely absent, with the body tapering to a fine point. All fins lack true spines.

Oarfish are so-called because of the paddle-like appendages at the end of their pelvic spines, which are used to help them balance and swim upright.