When the OZ-00MS Tallgeese mobile suit prototype proved too expensive and unfeasible for most pilots to handle, the Tallgeese's design was downgraded into a simpler, less-powerful unit. Naming their mobile suits after signs of the Zodiac, OZ's first mass-produced unit was called the OZ-06MS Leo. With its smaller size and lower performance, the Leo is both more economical to produce and easier for the average pilot to handle. The Leo would serve as the mainstay of the mobile suit forces for the Earth Alliance military, the Specials force, and later OZ and its Romefeller Foundation masters.

First introduced into service in AC 175, the Leo was slowly upgraded over the years leading up to the outbreak of total war in AC 195. By that time, the Leo had actually changed very little from earlier models. The primary reason for the Leo's long service record was unlike later models in the "Zodiac" series of mass-produced mobile suits, the Leo is highly flexible: in addition to a wide variety of hand weapons, with the appropriate equipment, it can be used in a variety of environments and tactical roles. The ground version of the Leo is typically armed with a 105mm machine gun, but it can also be outfitted with a beam rifle, a bazooka, or the powerful shell-firing cannon known as the dobergun. The optional shoulder-mounted shield stores a pair of beam sabers for anti-mobile suit combat. In addition, the Leo could be outfitted with a winged thruster pack for limited atmospheric flight capability.

Source: Gundamproject.com, New Mobile Report Gundam Wing