The Olympus OM System (O = Olympus, M = Maitani) was a line of 35mm single-lens reflex cameras, lenses and accessories sold by Olympus between 1972 and 2002 (some accessories were sold until early 2003).
The system was introduced by Olympus in 1972, more than a decade after Nikon, Canon, and other manufacturers had established their own SLR ranges. The range was designed by Yoshihisa Maitani, chief designer for Olympus, and his staff. The nucleus of the system was a series of compact bodies divided into a professional series and a later consumer-orientated series. The first model introduced was the all-mechanical M-1, which after pressure from Leica was renamed OM-1. The body included a full aperture TTL CdS exposure meter and a bayonet lens mount of relatively large diameter. By the end of the 1970s it was joined by the semi automatic OM-2 and consumer oriented OM-10. Olympus continued the naming pattern with the professional OM-3 and OM-4, and the consumer-level OM-20, OM-30, and OM-40. The system was accompanied by a series of Zuiko-branded lenses, as well as a generous selection of accessories. The majority of OM bodies and lenses were manual focus only; the OM-707 of 1986 was the only true autofocus model.
Material from http://en.wikipedia.org/wiki/Olympus_OM_system