'OB' (ii) was the designation of Allied convoys (together with a numerical and sometimes a literal suffix) plying from Liverpool out into the Atlantic (September 1939/August 1941).

The convoys of the 'OB' series sailed from Liverpool and comprised vessels sailing across the Atlantic to North and South America, and south through the Atlantic to Freetown in Sierra Leone, the Cape of Good Hope and thence the Indian and southern Oceans, together with coastal traffic. En route, vessels sailing from the ports of the Bristol Channel (Avonmouth, Barry, Newport and Cardiff) would join the convoy at a designated position, usually a few miles to the south of the Smalls Lighthouse. The convoys were initially routed to the south through St George’s Channel and then to the west for dispersal after the fall of night in a position about 865 miles (1390 km) to the west of Land’s End in the South-Western Approaches. Later, when bases in France had become available to the Germans, and to an altogether more limited extent the Italians, the convoys were routed round the north of Ireland from 11 July 1940 to enter the Atlantic via the North-Western Approaches.

At intervals, the convoys of the 'OB' series were designated with the suffix 'G' and were combined with the 'OA' series (also suffixed 'G') for direct passage to Gibraltar, or 'GF' to denote a fast convoy. The combined convoy was then coded as the 'OG' series.

The complementary 'OA' series of the same number sailed on the same date from Southend-on-Sea or, from 13 July 1940, from Methil on the northern side of the Firth of Forth.

In addition to their conjunction in the 'OG' convoys, the 'OA' and 'OB' series combined if they met each other at sea. The 'OA' series also carried a lot of local traffic from the Thames to the south coast ports of Southampton, Poole, Plymouth (Devonport), Fowey, Falmouth etc.

There were 346 such convoys before the 'OB' series was superseded in July 1941 by the 'ON' and 'OS' series to Halifax. Nova Scotia, and Freetown, SierraLeone, respectively.

The first of the convoys was OB.1 of 7/10 September 1939 with three British merchant vessels in the form of the 8,999-ton Athellaird, 6,791-ton Newfoundland and 7,087-ton Scottish Heather escorted by the destroyers Versatile and Vimy, and the last was OB.249 of 21 July/1 August 1941 with 45 merchant vessels and an eventual total of 16 escorts.

Of this series, the OB.293 convoy was notable inasmuch as it led to the loss of U-47 under the command of Kapitänleutnant Günther Prien, who had sunk the battleship Royal Oak in Scapa Flow shortly after the start of the war. The OB.293 westbound convoy of 37 ships, either in ballast or carrying trade goods, departed Liverpool on 2 March 1941 for ports in North America. The convoy was escorted by an escort group of the destroyers Verity and Wolverine, and corvettes Arbutus and Camellia under Lieutenant Commander J. M. Rowlands of Wolverine, which would stay with the convoy until it left the Western Approaches.

On 6 March the convoy was sighted and reported by U-47, which then shadowed the convoy as it was joined in the same day by three other boats, namely U-70, U-99 and U-A. During the night of the 6/7 March the wolfpack launched its attack. In the early hours of 7 March Kapitänleutnant Otto Kretschmer’s U-99 slipped into the convoy from ahead for a surfaced attack in which it sank the 6,568-ton British tanker Athelbeach and damaged the 20,638-ton British whale factory ship Terje Viken. Korvettenkapitän Joachim Matz’s U-70 sank the 5,258-ton British freighter Dunaff Head and damaged the 7,493-ton Dutch tanker Mijdrecht. The latter rounded on her attacker in a ramming attempt, the boat crash-diving to escape. Fregattenkapitän Hans Eckermann’s U-A , which had been built for Turkey before being taken over for German service, damaged a freighter.

The response of the escorts was swift and effective, and the U-boats therefore came under severe attack, the escorts firing more than 100 depth-charges over a five-hour period. U-A was damaged but was able to escape, U-99 escaped only by diving deep and waiting out the attack, and U-70 was damaged and forced to the surface, where it was sunk by the corvettes.

U-47 avoided damage and was able to remain in contact with the convoy, sending further reports and requesting reinforcements. Prien had also been able to torpedo Terje Viken, which was straggling after being damaged. The escorts attempted to bring her to port, but she finally sank on 14 March. At about 01.00 on 8 March Wolverine sighted a U-boat on the surface. She and Verity attacked, and after four hours drove the boat to the surface within yards of Wolverine. The boat then dived once more and the destroyer sent down a pattern of depth charges, which was rewarded with an underwater explosion. Wolverine was credited with the destruction of U-47, but analysis in the late 1990s suggests that the destroyer’s 'victim' was U-A, which was badly damaged but survived to reach port. No conclusion can be reached about the fate of U-47, which may have been lost to a diving accident.

The success of the defence of the OB.293 convoy, with the loss of the U-boat 'ace' Prien, coupled with the successful defence of the HX.112 convoy, and the loss of two more aces, Kretschmer and Kapitänleutnant Joachim Schepke, only one week later, marked a minor turning point in the Atlantic campaign.

Another milestone for the OB series of convoys was the battle for the OB.318 convoy, which saw the British capture of Kapitänleutnant Fritz-Julius Lemp’s U-110 together with a mass of signals intelligence that paved the way to the British breakthough into cracking the Germans' current Enigma naval code.

The OB.318 convoy of 38 ships, either in ballast or carrying trade goods, departed Liverpool on 2 May 1941 for ports in North America. The convoy was supported by Commander I. H. Bockett-Pugh’s 7^th Escort Group (10 warships led by the destroyer Westcott), which was joined in mid-ocean by Commander J. Baker-Cresswell’s 3^rd Escort Group (eight warships led by the destroyer Bulldog). Opposing them was a force of 19 U-boats, though in the event only six were in a position to attack.

Kapitänleutnant Herbert Kuppisch’s U-94 spotted and reported the convoy, and started to shadow it as Vizeadmiral Karl Dönitz, the Befehlshaber der U-Boote, alerted other U-boats in the area. Six boats were within striking distance and began to close on U-94.

On the same day the escort was joined by five ships from Iceland and the destroyers of the 3^rd Escort Group, which was to take over escort duties between the Western Approaches to the dispersal point south of Greenland. Three ships and the destroyers of the 7^th Escort Group left for Iceland during 7 May, leaving the escort still at 10 warships. At the fall of night on 7 May U-94 attacked the convoy, sinking two ships. Kuppisch was able to introduce his boat into the convoy by submerging ahead of the convoy and letting the lead escorts pass over his boat, and was then in a position to fire at close range on the ships in the centre of the convoy, sinking the 10,000-ton British Ixion and 5,658-ton British Eastern Star. U-94 was located by the sloop Rochester, and was counterattacked by her, Bulldog and Amazon for four hours, sustaining sufficient damage to force the retirement of the boat until repairs could be effected and its patrol resumed.

On 8 May the remaining vessels of the 3^rd Escort Group (three corvettes and two armed trawlers), together with the 16,688-ton armed merchant cruiser Ranpura, joined the convoy and the 7^th Escort Group’s remaining ships departed. During the evening of 8 May U-110 and U-201 also made contact, tracking the convoy until the following morning. In an unusual move, the captains of the two boats made rendezvous in order to co-ordinate their attack: Lemp in U-110 was to make a submerged attack from ahead while Kapitänleutnant Adalbert Schnee in U-201 was to do the same but from the rear. It was expected that the escort would have departed by this time, leaving the convoy vulnerable to their assault.

During the morning of 9 May U-110 began its attack. Lemp was surprised to find escorting warships still present, but succeeded in penetrating the convoy to sink two British ships, the 2,609-ton Bengore Head and 4,976-ton Esmond. U-110 was then counterattacked by Aubretia, Bulldog and Broadway, forced to the surface and abandoned. U-110 was still afloat and was captured by the three warships, which dropped out of the convoy to secure their prize. Meanwhile Schnee in U-201 also attacked, sinking the 5,802-ton British Gregalia and damaging the Empire Cloud before being counterattacked by Amazon, Nigella and St Apollo, damaged and and forced to retire.

Early on 10 May the OB.318 convoy was spotted by Kapitänleutnant Herbert Wohlfarth’s U-556, which immediately attacked and damaged the British Aelybryn. Having reached its dispersal point, the convoy divided during the night, while the escorts departed to meet their next charge. U-556 was able to keep in touch with a group of ships heading to the south-west, and during the day sank the 4,861-ton British Empire Caribou and 5,086-ton Belgian Gand. During the following two weeks the OB.318 convoy’s remaining 36 ships reached ports along the eastern seaboard of North America, though not before three, including the convoy commodore’s ship Colonial, had been sunk by U-boats patrolling near their destinations.

The OB.318 battle can be categorised as a limited Allied victory in matériel terms: five ships were lost in attacks on the convoy, and another two after dispersal, while one U-boat was sunk and another two damaged. Altogether more significant, though, was the capture of U-110 and its communications equipment, including an Enigma coding machine, current rotors and codebooks. The U-boat later sank while under tow, but this vitally important treasure trove had already been salved and was passed to the 'Ultra' codebreakers at Bletchley Park, where it paved the way to a lasting breakthrough in reading German naval traffic (with some interruptions) throughout the rest of the Atlantic campaign.