The Arnold Hague database for the OA series of convoy: The OB series of mercantile convoy sailed from Liverpool into the Atlantic to be dispersed about about 750 nautical miles west of Lands End. At intervals, the OB convoy would join with the complementary
OA series (sailing from Liverpool) to sail to Gibraltar. In this case both OA and the OB would be designated "G" and become the OG series. Even when dispersing in the Atlantic, the OB convoy would combine with the OA convoy if they met at sea. The OB series ran from September 1939 until July 1941 when it was replaced by the OS series to Freetown and the ON series to Halifax.