The ON series of mercantile convoy replaced the OB series in July 1941 and continued until May 1945. Initially it sailed from Liverpool and was dispersed about 750 nautical miles west of Lands End as the OB convoys were. Later, as escorts became available (March 1942), it was escorted all the way to ports in Canada or the USA. From October 1944 sailings also started from Southend with the Liverpool section joining at sea.

ON.185 was cancelled.