When the 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 ("Leo" being the fifth Zodiac sign meaning "lion" - perhaps in reference to the lion symbol of the Romefeller Foundation). 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 , the Leo was slowly upgraded over the years leading up to the outbreak of total war in . By that time, the Leo had actually changed very little from earlier models. The primary reason for the Leo's long service record was its simple adaptability: in addition to a wide variety of hand weapons, the Leo could be outfitted with a winged thruster pack for limited atmospheric flight capability. Other Leo variants include:
- Leo Early Type
- An older version of the OZ-06MS, the Leo Early Type was almost completely identical to the standard unit used in AC 195, with only minor (cosmetic) differences. Despite its age, this version of the Leo was still in use in more remote areas of Earth, including Colonel Bunto's breakaway nation in former China.
- Leo Cannon Type
- An upgraded version of the OZ-06MS, the Leo Cannon Type was identical to the standard model used in AC 195 except for the addition of a pair of beam cannons on the shoulders. The Leo Cannon Type was usually us
- Leo Space Type
- Requiring a mobile suit force to help police (read "blockade") travel and commerce between the space colonies, OZ and the Earth Alliance adapted the standard Leo for space use with the addition of a booster backpack equipped with rocket and vernier thrusters. This Leo variant also carried a beam rifle similar to that of the standard Leo, except that it was fitted with a radar disc for targeting in place of the usual optical sensor.
- EWAC Leo
- A variant only existing in the G-Unit side story, the OZ-06MS EWAC Leo (unofficial name) was a Leo modified to carry a high-powered radome in the head section of the mobile suit. The suit is lightly armoured, and carries a customised machinegun as its primary weapon. Objects mounted to the left shoulder may be additional weapons, but their nature or use was not indicated in either appearance.