The GCR 8K was a class of 2-8-0 freight locomotives designed by John G. Robinson, basically a superheated variant of his 8A with a leading axle. A total of 126 were built between 1911 and 1914. 56 of these had been built in the CGR's Gorton works, 20 by Kitson and 50 by North British. In World War I, this design was slightly modified and built for wartime use as ROD 2-8-0. A total of 521 had been completed when the war ended. Shortly after the war, the GCR built 19 of a variant with a slightly larger boiler as class 8M.

In 1923, the LNER inherited 128 of the class 8K and 17 of the 8M, since two 8M had been rebuilt into 8K in 1922. The 8K became class O4 and the 8M became class O5. Of the ROD 2-8-0, 273 were still remaining in Britain and were now also integrated into class O5. The LNER continued to rebuild the O5, or the former 8M, into the O4. It took until 1946 that all had been rebuilt.

In World War II, 92 locomotives were brought to the Middle East for wartime service and never returned. Between 1944 and 1949, 58 were rebuilt by Thompson into his class O1. After different smaller rebuilds, the LNER designated the basic variant O4/1 and the rebuilt ones became O4/2 to O4/8. British Railways took over 329 in 1948 and withdrew them between 1958 and 1966. The only one of the non-ROD locomotives that was preserved is 63601. It is still in working condition and operates on the preserved Great Central Railway.