From 7 September 1939, shortly after the outbreak of the Second World War, all traffic outbound from Britain was organized into the OA and OB series, sailing from London via the English Channel (OA) and from Liverpool via St Georges Channel (OB) into the South-Western Approaches.
After the fall of France in June 1940 these were successively reorganized as German aircraft, submarines and surface ships reached further and further into the Atlantic, until ships formerly assigned to OA/OB convoys were formed into ON convoys sailing from Liverpool via the North Channel and escorted all the way to Halifax Harbour.
These figures do not include stragglers; although the majority of casualties to U-boats were ships that had fallen out of convoys or were sailing independently.
This situation, which has proved confusing to modern historians, prevailed until a new and separate series of ONS (Outbound North Slow) convoys was organised.
The ONS series were suspended in the summer of 1944 as escort groups were diverted to cover the Normandy landings.