The Great Northern Railway (GNR) Class O1 was a class of two-cylinder 2-8-0 steam locomotive designed by Nigel Gresley for heavy freight work and built by the GNR between 1913 and 1919.

Gresley designed the O1 2-8-0 for the heavy coal trains on the mainline from Grantham to London, ordering five examples 1913, which were completed at Doncaster in 1914.

A further fifteen were ordered in January 1916, but due to the First World War delays were incurred and construction was transferred to the North British Locomotive Company, who delivered the first ten in April 1918, followed by a further five in October and November 1919.

The class was re-designated O3 by the London and North Eastern Railway in 1944 to release the O1 classification for use with the LNER Thompson Class O1 introduced during the Second World War.

Seventeen examples survived into British Railways ownership in 1948, but all had been withdrawn by 1952 and scrapped after World War 2 .