Because of wartime restrictions on new-build locomotives, they were rebuilds of LNER Class O4 "ROD" 2-8-0s built before and during World War I, although most of the locomotive was replaced during this rebuild. The first LNER rebuild took place in February 1944, and 58 locomotives were rebuilt to class O1 in total, with the last being locomotive 63856 in October 1949 during the early British Railways era, after which the programme was halted. The main modification to the original Great Central Railway Class 8K design was the incorporation of a standard LNER 100A boiler, Walschaerts valve gear and new cylinders.
The locomotives passed to British Railways (BR) Eastern and North Eastern Regions on 1 January 1948 and were given BR running numbers in the range 63570-63920. However, this range included many unrebuilt O4s.
63760 at Gorton loco shed on 8 November 1958. This O1 is unusual in being fitted with two air pumps just in front of the cab for operation of heavy iron ore trains from Tyne Dock to Consett steel works.