OM was formed in 1899 through the merger of two companies — Grondona Comi & C and Miani Silvestri & C — as Società Anonima Officine Meccaniche, initially to manufacture railway rolling stock. Car production did not begin until 1918, when OM acquired the Brixia-Züst plant in Brescia, absorbing the Zust car company. The first OM car, the Tipo S305, was derived from an existing Zust model and fitted with a 4,712 cc inline four-cylinder side-valve engine.
Further passenger models followed in quick succession. The Tipo 465 of 1919 used a 1,327 cc engine; the Tipo 467 (1,410 cc) and Tipo 469 (1,496 cc) appeared in 1921. The model names reflected the cylinder count and bore diameter: four cylinders, bore of 65, 67, or 69 mm respectively.
The most significant car was the Tipo 665 "Superba," introduced in 1923. Based on the earlier four-cylinder architecture, it used a 2-litre six-cylinder engine that proved highly competitive in motorsport. Some examples were fitted with Roots-type superchargers; later variants were bored and stroked to 2.2 and 2.4 litres, retaining the original model designation. In 1925 and 1926 the Superba achieved top five finishes in the 2-litre class at Le Mans, building the marque's international racing reputation ahead of its defining moment two years later.
The inaugural Mille Miglia race of 1927 stands as OM's greatest achievement. Ferdinando Minoia and Giuseppe Morandi led the field to complete an OM 1-2-3 finish — the first three positions across the line, all taken by the same marque. They completed the course at an average speed of 77.7 km/h (48.27 mph) over 21 hours, 4 minutes, and 48 seconds. For an Italian manufacturer to sweep the podium of Italy's new great road race in its inaugural running was a result of profound commercial and sporting significance.
From 1925 OM began building trucks and buses using licensed Swiss Saurer engines and mechanical components, a partnership that persisted throughout the rest of the company's history. Passenger car production tapered off in the late 1920s and effectively ended around 1932, though existing stocks continued to be sold for another year or two. In total approximately 7,500 OM automobiles were built over roughly fifteen years.
A final car appeared as a prototype in 1934: the OMV, also called the Alcyone, which featured hydraulic brakes, a full synchromesh transmission, and an engine with overhead exhaust valves. It never reached production.
Fiat took over OM in 1933, and from 1934 passenger car sales ceased definitively. OM became a commercial vehicle and railway components manufacturer within the Fiat group. In the post-World War II era the company's main new product was the Leoncino (1950), a 3.0 to 3.5 tonne light truck that became an immediate success and spawned a family of successors including the Tigrotto, Tigre, Lupetto, Cerbiatto, and Daino, launched between 1957 and 1964.
In 1968 OM was definitively merged into Fiat's Commercial Vehicles division, which also encompassed Fiat and Unic. In Switzerland, Austria, France, and Germany, OM trucks of the 1960s and 1970s were sold as Saurer-OM, Steyr-OM, Unic-OM, and Büssing-OM respectively. In 1975 the brand was absorbed into Iveco and gradually disappeared from the truck and bus market, though the OM name survived as an independent forklift manufacturer.