The OA-93 was the FIRST designated AR-15 pistol according to Olympic Arms. It is piston driven and doesn't require a buffer tube system giving it one of the shortest profiles for an AR platform available at 17" overall and 4.46 pounds.
These were made from 1993 until outlawed in 1994 and then after the Assault Weapons Ban expired from 2004-2007. This was made its final year 2007 and is highly collectible due to its unique design and overall size.