The Owl was created by Stan Lee and Joe Orlando and first appeared in Daredevil volume 1 - Issue #3 (Aug 1964). Vince Colletta was the inker and Sam Rosen provided the lettering. The cover artwork was done by Jack Kirby and Artie Simek with assistance from Mr. Colletta.
The Owl was the very first original villain created for the title. In issue #1, Daredevil encountered a gangster called "The Fixer", who died at the end of the story. And in issue #2, he battled Electro, who was created as an adversary for Spider-Man.
Leland was a wall-street wizard and got the nickname Owl by his peers. The IRS started snooping and revealed his ties to the underworld. Depressed, he took a serum that gave him the ability of flight. He has had several clashes with Daredevil, Spider-Man, Black Widow, and even Doctor Octopus. Through the years, the serum has caused him to mutate further. He has recently resurfaced to take revenge on Daredevil. The Hood later seemingly killed Owl in order to gain respect among the figures of New York's criminal underworld and steal Deathlok from him.
Later, it turns out Owl survived the attack, as he is under police custody, but gets freed by Kingpin's henchmen shortly - in return Wilson Fisk wants Owlsley's services in order to distract Daredevil and takeover The Hand. That leads to capture of Dakota North, who Owl plans to torture, but his plot gets ruined once again by Daredevil, who shows no mercy to his long time nemesis - he cuts Owl's arms and legs with a sword, very likely crippling him by doing so. This also lead to the beginning of Shadowland event.
Later, it appears that he is fully healed and is in possession of the head of Silvermane which Boomerang's Sinister Six tries to steal.
Powers, Abilities & Paraphernalia
The Owl's most distinctive power is his capacity for limited flight. The source of this ability is a special serum he has ingested which originally enabled him to naturally glide for short distances (roughly 100 feet); ultimately, his physical makeup has changed to a degree which allowed him to expand on these initial facilities. While there may be a primary physical source at least assisting the Owl's abilities, his glide/flight ratio (the rate of drop to the distance traveled) suggests the presence of some type of psionic ability. Typically, he must to leap from a height of at least 20 feet (6.1 m) off of the ground to allow for enough lift, but he has occasionally started from a standing position - although he has not been seen to travel higher than 15 feet in this way. His distance and speed are variable depending on ambient air currents and the height of his original lifting leap. With a good tailwind he can travel distances of several hundred yards and reach speeds of up to 30 miles per hour. Diving from a helicopter or skyscraper could increase his rate of flight even further, while hopping from a parked city bus or freeway overpass would result in a slower and shorter glide. The Owl could possibly increase his flight efficiency through other methods, such as leaping from a moving vehicle. It is as yet unknown how high the Owl can travel before his breathing would become impaired, but his rate and speed will decrease as he loses altitude to assist in landing. The Owl has managed to achieve better controll of his flying abilities through use over time. While airborne, he uses his arms and legs for balance and steering but is still able to perform complex midair maneuvers and engage in combat with little variation in his flight pattern (such as a free-fall followed by a quick climb in altitude, allowing for a "swoop" attack). The Owl's powers are affected by his physical condition at the time, and any illness or injury would impede his ability to use them. Aside from the flight serum, the Owl has used other experimental drugs and unique surgical procedures to enhance his flying ability through the years. The exact nature of these methods and their relative effectiveness has yet to be revealed.
It's a common misconception that Owl is a mutant, but he is a mutate who obtained his powers by experimenting on himself.
Earth 295: Age of Apocalypse
In the Age of Apocalypse reality, Owl was a member of the Marauders (Human terrorists enhanced by Apocalypse), along with teammates Arcade, Kingpin, and Red (aka Norman Osborn). The four villains were sent by Apocalypse to destroy the land of Wakanda. Though they were able to murder Black Panther and destroy much of the land, Owl's life was cut short by Hawkeye, who blew him up with an explosive missile.
It is assumed that this Owl had the same powers as the one in the regular Marvel Universe (Earth-616). He also was equipped with wrist weapons and "air-steppers"; boot attachments that allowed him near-flight.