The railway administrations of the Tsarist Empire combined eight-coupled freight locomotives under the letter О, which made it to a number of more than 9,000 in different versions. The series went back to a type from Maschinenfabrik Esslingen, which had been supplied to several railway administrations from 1878 onwards. In order to follow the trend of the time, the Vladikavkaz railway had two-cylinder compound locomotives based on this type manufactured in Kolomna in 1890, which marked the beginning of the О series.
Oв No. 324 in August 2015 at the Podmoskovnaya depot in Moscow
From 1893 a unified standard type was manufactured, which had a wheel diameter of only 1,150 mm and cylinders with a diameter of initially 500 and 710 mm with a stroke of 650 mm. After 129 examples were produced, the diameter of the low-pressure cylinder was increased to 730 mm. A total of 1,373 examples of the standard type from 1893 were manufactured by 1899, which were also supplied from Germany and Austria due to the limited capacities of the Russian factories.
Armoured locomotive O? No. 5067 in the Central Museum of the Russian Armed Forces
In 1897 a new standard type appeared, of which a total of 3,172 were built by eight Russian factories by 1903. This variant, designated Од, now had a driver diameter of 1,200 mm and a boiler pressure increased from 11 to 11.5 bar.
After two prototypes were completed in 1899 with a Heusinger type valve gear instead of the problematic Joy valve gear, a total of 4,178 examples were built between 1901 and 1907, which also had a Heusinger valve gear. The diameter of the cylinders had been increased to 510 and 740 mm. The two types were designated Ов and Оk and were alternatively known as the 1901 standard type and the 1904 standard type. Between 1908 and 1915 another 190 examples of the Ов were made.
When superheated steam became established in locomotive construction, a few locomotives with superheaters and simple expansion were built between 1908 and 1915. These were given the designation Оп and soon other examples of the earlier series were rebuilt in the same way. After the October Revolution, another 72 pieces were made between 1925 and 1928. Later, some locomotives received boilers with a pressure of 14 bar during conversions. After the introduction of more powerful freight locomotives from the 1930s, they were increasingly used for shunting. The last operations on the route took place in 1964 on a branch line of the Trans-Baikal.