The common name oarfish is presumably in reference to either their highly compressed and elongated bodies or the shape and use of their pelvic fins. The family name Regalecidae is derived from the Latin regalis, meaning "royal". The occasional beachings of oarfish after storms, and their habit of lingering at the surface when sick or dying, make oarfish a probable source of many sea serpent tales.
Although the larger species are considered game fish and are (to a minor extent) fished commercially, oarfish are rarely caught alive; their flesh is not well regarded due to its gelatinous consistency.
The tapering, ribbony silver bodies of oarfish—together with an impressive, pinkish to cardinal red dorsal fin—help explain the perception of majesty taken from rare encounters. The dorsal fin originates from above the (relatively small) eyes and runs the entire length of the fish. Of the ca. 400 dorsal fin rays, the first 10–12 are elongated to varying degrees, forming a trailing crest embellished with reddish spots and flaps of skin at the ray tips. The pelvic fins are similarly elongated and adorned, reduced to 1–5 rays each. The pectoral fins are greatly reduced and situated low on the body. The anal fin is completely absent and the caudal fin may be reduced or absent as well, with the body tapering to a fine point. All fins lack true spines. At least one account describes the oarfish as giving off "electric shocks" when touched.
Like other members of its order, the oarfish has a small yet highly protrusible oblique mouth with no visible teeth. The body is scaleless and the skin covered with easily abraded, silvery guanine. In the streamer fish (Agrostichthys parkeri), the skin is clad with hard tubercles. All species lack gas bladders and the number of gill rakers is variable.
Oarfish coloration is also variable; the flanks are commonly covered with irregular bluish to blackish streaks, black dots, and squiggles. These markings quickly fade following death. The king of herrings is by far the largest member of the family at a published total length of 11 meters (with unconfirmed reports of 15 meters or more) and 272 kilograms in weight. The streamer fish is known to reach 3 meters total length whilst the largest recorded specimen of Regalecus russelii measured just 5.5 centimeters standard length. It is probable that this little-known species can regularly reach a maximum length of at least 15.2 meters (50 ft).
Rare encounters with divers and accidental catches by trawls have supplied what little is known of oarfish behavior and ecology. Apparently solitary animals, oarfish may frequent significant depths from 20–1,000 meters. In Bermuda, Teddy Tucker, a fisherman and treasure hunter, has reported their surfacing at night when lights were at the surface. It is possible that the light attracted them.