'ONS' was the designation of Allied slow convoys (together with a numerical suffix) plying the route from Liverpool in the UK to Halifax, Nova Scotia, as the slow element of the 'ON' series, and as such successors to the 'OBS' series and reciprocals of the 'SC' series (March 1943/June 1945).

There were 51 such convoys. Until April 1943, ships capable of speeds between 9 and 13 kt were assigned to the odd-numbered (fast) 'ON' convoys, which were sometimes designated 'ON(F)', while ships capable of speeds between 6 and 9 kt were assigned to even-numbered (slow) convoys, which were sometimes designated 'ON(S)' or, somewhat ambiguously, 'ONS'. This situation lasted until a separate series of 'ONS' (Outbound North Slow) convoys was organised. These were numbered sequentially from ONS.1, which departed on 15 March 1943 with 39 merchant vessels and an eventual total of 19 escorts to arrive on 4 April, to ONS.51, which departed on 21 May 1945 with 19 merchant vessels and an eventual total of 11 escorts to arrive on 4 June.

ON.171 and all following convoys of this series were fast convoys. The 'ONS' series was temporarily suspended in the summer of 1944 as all the escort groups were diverted to cover 'Neptune' (iii) and 'Overlord'.

In overall terms, 1,873 ships sailed in 51 'ONS' convoys, of which only five came under attack, in two cases resulting in battles of some significance: the ONS.5 convoy is regarded as the turning point of the Battle of the Atlantic, and ONS.18 was the last major convoy battle. The 'ONS' convoys lost only 19 ships. Of the notable battles associated with the 'ON' and 'ONS' convoys, of which 40 lost six or more ships, eight were in the 'ON' series (three fast and five slow) and one in the 'ONS' series.

The ONS.5 convoy was attacked in a week-long battle between the end of April and beginning of May 1943, and lost 12 ships, while six U-boats were sunk. This started the period known to German submariners as schwarz Mai (black May), generally regarded as the turning point in the Battle of the Atlantic. Attacked in September 1943, the ONS.18 and ON.202 convoys lost six ships and three escorts in exchange for the destruction of three U-boats.

The battle for the ONS.5 convoy in May 1943 marked the decisive turning point in the Battle of the Atlantic when the preponderance of changed from German to Allied superiority. The battle surged back and forth for a week, and involved more than 50 Allied ships and their escorts, and more than 30 U-boats, each side suffering heavy losses. The ONS.5 convoy was almost the last Allied convoy to suffer heavy losses, however, and the level of losses suffered by Grossadmiral Karl Dönitz’s U-boat arm marked the start of an apparently inescapable trend toward this level of losses for the rest of the campaign.

The ONS.5 convoy of 42 ships departed Liverpool on 21 April 1943, and was protected by Commander P. W. Gretton’s British Escort Group B7 (destroyers Duncan and Vidette, frigate Tay, and corvettes Loosestrife, Pink, Snowflake and Sunflower) together with the armed trawlers Northern Gem and Northern Spray as rescue ships, and the replenishment oiler British Lady. The convoy was joined by other escort vessels as the battle progressed. ONS.5 was just only one of the several Allied convoys at sea at the end of April: in the Western Approaches were also the departing ON.180 and arriving HX.234, just arriving; approaching North America were ONS.4 and ON.179; departing North America was SC.128; and in mid-Atlantic, due to pass ONS.5 east of Greenland, was SC.127, and two other eastbound convoys, HX.235 and HX.236 were also in mid-Atlantic on a more southerly route.

Thus there were more than 350 merchant ships on passage through the North Atlantic at this time. Ranged against them were 58 U-boats in 3 patrol lines: the 'Specht' wolfpack of 17 boats to the south of Greenland on the western side of the mid-Atlantic 'air gap', the 'Meise' wolfpack of 30 boats to the east of Greenland covering the northern route, and the 'Amsel' wolfpack of 11 boats to the south of the 'Meise' wolfpack to cover the southern route.

The 'Meise' wolfpack had been deployed to catch SC.127, which had been identified by the B-Dienst radio intercept and decryption service, but on 26 April SC.127 had slipped through a gap in the line and escaped undetected. Realising on the following day what had happened, and aware that a slow westbound convoy was imminent, Dönitz ordered the revision of the 'Meise' wolfpack: the 16 easternmost U-boats formed the patrol line as the 'Star' wolfpack to intercept it, and at 08.00 on 28 April U-650 sighted and reported the ONS.5 convoy. The 'Star' wolfpack quickly gathered to attack it.

This was not the start of the convoy’s history, however, for on the night of 21/22 April, shortly after departing Liverpool, the freighter Modlin had to turn back with engine trouble. On 24 April, in a moderate gale, the air escort from Scotland, spotted and sank Oberleutnant Dietrich von Carlewitz’s U-710 only 10 miles (16 km) ahead of the convoy but probably unaware of it. On 26 April the storm-damaged freighter Penhale detached to return to Reykjavík escorted by Northern Gem, which rejoined the following day. On the same day Vidette joined with three merchant ships from Iceland.

On 28 April the convoy entered the 'Star' wolfpack’s patrol area, and was sighted and reported by U-650, and by the fall of night this boat had been joined by U-375, U-386, U-528 and U-537, and at midnight these boats began their attack. Warned by the radio chatter of the U-boats' presence, Gretton mounted a vigorous defence, and during this first night none of the Allied ships was hit while U-386 and U-532 were damaged and forced to return to base: U-386 reached St Nazaire on 11 May, but Fregattenkapitän Ottoheinrich Junker’s U-532 was attacked and sunk by aircraft in the Bay of Biscay during the same day.

The German attack on the convoy was pressed right into the daylight hours. At 12.000 on 29 April the 6,198-ton US freighter McKeesport was torpedoed and sunk, but the attacker, Kapitänleutnant Wilhelm von Mässenhausen’s U-258, was also damaged had had to return to base. On this same day the Admiralty arranged reinforcements for the ONS.5 convoy’s escort: the destroyer Oribi was detached from the SC.127 convoy’s escort to arrive on 30 April, while Captain J. A. McCoy’s British 3^rd Support Group (destroyers Impulsive, Offa, Panther and Penn) was to arrive on 1 May.

During the course of the next few days the action was dictated by the weather rather than any tactical considerations. In worsening conditions the ONS.5 convoy found itself making less than 3 kt into a Force 10 gale. The weather started to scatter the convoy, some of the ships being driven 30 miles (50 km) from the convoy, and the escorts were kept busy rounding up stragglers. On 30 April the first reinforcements arrived, easing the escort vessels' task. The weather made refuelling impossible, and a number of the destroyers became so low on fuel that it became problematical whether or not they would be able to continue. On 3 May Gretton was forced to take Duncan to St John’s, Newfoundland, at her most economical speed of 8 kt, and arrived with just hours of fuel left. In Gretton’s absence command was assumed by Lieutenant Commander R. E. Sherwood, Tay's captain. Later on that night Impulsive also detached to Iceland, together with Northern Gem carrying the survivors from McKeesport, while Panther and Penn were detached to Newfoundland.

Throughout this period the U-boats tried to maintain contact, though any attack was impossible. Only six of the 'Star' wolfpack’s boats had been able to make contact, and of these three had been forced to return to base. Dönitz decided that nothing could further be gained, and on 1 May ordered boats from the 'Star' and 'Specht' wolfpacks, together with some newcomers, to form a new patrol line farther to the west as the 'Finke' wolfpack, which had taken up station by 3 May with no fewer than 27 boats. Their primary task was the interception of the westbound SC.128 convoy. This German reorganisation was not completed without incident, however, for on 1 May two of the newcomers were attacked by Allied aircraft in separate incidents: one was thought at the time to have been Oberleutnant Werner Winkler’s U-630 and was sunk, but the boat in question is now believed to have been Kapitänleutnant Heinrich Brodda’s U-209, which was damaged in the attack and later sank while attempting to return to base; the other was U-438, which was only slightly damaged in the attack.

By 4 May the weather had abated to Force 6, and the ONS.5 convoy was was able to make 6 kt, though it had been reduced to 30 merchant ships and seven escorts. The convoy’s other ships were scattered and proceeding independently, including a group of four with Pink, some 80 miles (130 km) astern of the main body. At 12.00 on 4 May, the ships were sighted and reported by U-628 of the 'Finke' wolfpack, and identified as the ONS.5 convoy rather than the expected SC.128 convoy. The wolfpack’s boats began to gather from their extended patrol line, and by evening were ready to make their attack. The assault started at the fall of night and continued throughout the dark hours, during which five ships were torpedoed in surface attacks while several U-boats were damaged. The assault continued into the day, the U-boats switching to submerged attacks, and another four ships were lost.

The losses included the 4,737-ton British freighter Lorient to Kapitänleutnant Ulrich Folkers’s U-125, 4,635-ton British freighter North Britain to Oberleutnant Günter Gretschel’s U-707, 5,081-ton British freighter Harbury to Kapitänleutnant Heinrich Hasenschar’s U-628, 5,561-ton US freighter West Maximus to Kapitänleutnant Ulrich Graf von Soden-Fraunhofen’s U-264, 4,586-ton British freighter Harpurley to U-264, 2,864-ton British freighter Bristol City to Kapitänleutnant Rolf Manke’s U-358, 5,512-ton British freighter Wentworth to U-358, 5,507-ton British freighter Dolius to Kapitänleutnant Oskar Staudinger’s U-638, 5,136-ton British freighter Selvistan to Kapitänleutnant Ralf von Jessen’s U-266, 5,306-ton British freighter Gharinda to U-266 and 1,570-ton Norwegian freighter Bonde to U-266.

Several boats also found Pink's group and attacked, Kapitänleutnant Joachim Deecke’s U-584 sinking the 5,565-ton US freighter West Makadet. Pink counterattacked and was credited with the destruction of Oberleutnant Werner Happe’s U-192, though the boat is question is more likely to have been Manke’s U-358, which was damaged.

Dönitz ordered the attack to be pressed into the night but, as a thick fog formed the advantage passed to the escorts. The U-boats lost visibility in the fog, while the radar-equipped escorts were able to located and attack the U-boats without hindrance. Thus the Allies lost no ships, but the Germans lost Staudinger’s U-638 to Loosestrife, Folkers’s U-125 to the combination of Oribi and Snowflake, and Kapitänleutnant Herbert Neckel’s U-531 to Vidette.

On the morning of 6 May, Allied reinforcement arrived in the form of Commander G. N. Brewer’s 1^st Support Group (sloop Pelican and frigates Jed, Spey and Wear). The group announced its arrival by attacking and sinking Korvettenkapitän Heinrich Heinsohn’s U-438, while the ex-US Coast Guard cutter Sennen, sent to help Pink's group, made an attack which was also credited as a kill though later found to have been unsuccessful. The escort now had a strength of 13 warships.

The 'Finke' wolfpack had clearly outlived its utility, and faced mounting losses if its effort were continued. On 6 May, therefore, Dönitz called off the assault and ordered the 'Finke' wolfpack to retire.

Although it was still a week from its destination, the ONS.5 convoy was not attacked again, which was fortunate for the Allies as on 6 May Offa and Oribi detached to refuel, while the warships of the Escort Group B7, short of fuel and ammunition, were poorly placed for further action, leaving only the ships of 1^st Support Group in any condition to fight. In the course of one week, the ONS.5 convoy had been attacked by more than 40 U-boats and lost 13 ships (63,000 tons), while the escorts had sunk six U-boats, and seriously damaged another seven. This battle demonstrated that the convoy escorts had mastered the art of convoy protection. The weapons and expertise at their disposal meant that henceforth they would be able not only be able to protect their charges and repel attack, but at the same time to take the offensive and inflict significant losses on the U-boats. The ONS.5 convoy thus marked the turning point in the Battle of the Atlantic, and after this the Allies inflicted a series of defeats and heavy losses on the U-boat arm in schwarz Mai, which led Dönitz to withdraw his forces from the North Atlantic arena to rethink their tactics and make further boats available.

The Germans returned to the fray in a fresh offensive during the autumn of 1943, when the first major battle was the U-boat attack on the ONS.18 and ON.202 convoys in September after the Germans had adopted new tactics and improved the technology of their boats. This allowed Dönitz to despatch the 'Leuthen' wolfpack of 21 boats in the hope of a return to an earlier level of success. The wolfpack established itself in a patrol line to the south of Greenland in a position to intercept westbound convoys as they were about to enter the 'air gap' in which the Allies could not fly air patrols as it was too far from land bases. The 'Leuthen' wolfpack was to attack any convoy it encountered while crossing the gap, before breaking off to reverse course and attack eastbound convoys.

On 12 September the ONS.18 convoy of 27 ships departed Liverpool with the escort of Commander M. B. Evans’s British Escort Group B3 (destroyers Escapade and Keppel, frigate Towey, and corvettes Narcissus, Orchis and Free French Lobélia, Renoncule and Roselys). The convoy was also accompanied by the merchant aircraft carrier Empire MacAlpine, which was a grain carrier adapted to carry four fighters.

When Admiral Sir Max Horton’s Western Approaches command became aware of the 'Leuthen' wolfpack’s presence, it decided to reinforce the ONS.18 convoy effort: the following ON.202 convoy was ordered to close up, and Commander C. E. Bridgman’s 9^th Support Group (Canadian destroyer St Croix, British frigate Itchen, and Canadian corvettes Chambly, Morden and Sackville) was sent to join the force. The ON.202 convoy had departed Liverpool with 38 merchant ships escorted by Lieutenant Commander P. W. Burnett’s Canadian Escort Group C2 (destroyers Gatineau and British Icarus, British frigate Lagan, and corvettes Drumheller, Kamloops and British Polyanthus). Thus 65 ships were escorted by 19 warships against the likelihood of attack by up to 21 U-boats.

On 19 September the ONS.18 convoy was sighted by Kapitänleutnant Paul-Friedrch Otto’s U-270 which sent a sighting report and was then authorised to attack. U-270 fired an acoustic-homing torpedo at Lagan, damaging her stern, in the first case of an Allied warship being damaged by this new weapon. The escorts counterattacked, but U-270 escaped, and Escapade was damaged by a misfire from her Hedgehog anti-submarine mortar. Both Lagan and Escapade were forced to detach, returning to base under escort.

The boats of the 'Leuthen' wolfpack closed on the combined convoys during 19 September, but two of them were attacked by air patrols which, since the introduction of a number of VLR (Very Long Range) Consolidated Liberator maritime patrol bombers during the summer, had been extended into the 'air gap'. Oberleutnant Dietrich Schöneboom’s U-341 was attacked and sunk by a Liberator of the RCAF’s No. 10 Squadron, and Kapitänleutnant Mafred Kinzel’s U-338 was attacked by a Liberator of the RAF’s No. 120 Squadron, which was credited with a kill, though post-war analysis showed that the boat was only damaged by this attack and was later engaged and sunk by the corvette Drumheller. On the night of 19/20 September several of the 'Leuthen' wolfpack’s boats made contact with the convoy. U-260 attacked without success, while Kapitänleutnant Horst Hepp’s U-238 sank the 7,176-ton US freighter Theodore Dwight Weld and damaged the 7,176-ton US freighter Frederick Douglass. The latter fell out of the convoy, and was sunk later in the same day by Oberleutnant Otto Ferro’s U-645.

On 20/21 September about 12 boats were in contact, of which eight made attacks. Kapitänleutnant Rudolf Bahr’s U-305 torpedoed St Croix, which sank with the loss of 66 of her crew. Kapitänleutnant Oskar Curio’s U-952 torpedoed Polyanthus, which exploded and sank with all but one of her crew, who was rescued, along with 81 of St Croix's crew, by Itchen. U-386 was damaged in a depth-charge attack and forced to retire. U-603 was ordered to rendezvous but could not do so as a result of air attacks.

The 'Leuthen' wolfpack’s boats remained in contact, and on the night of 21/22 September renewed their attack. U-377 attacked and claimed hits, but these were not confirmed, and the boat was then attacked and damaged by aircraft, and forced to retire. U-230 attacked, but again gained no confirmed hits and was attacked by an unidentified escort and forced to retire. Oberleutnant Robert Schetelig’s U-229 was attacked and destroyed, this being credited to Keppel. U-422 was damaged by air attack, but could continue.

On 23 September the convoys reached the area of the Grand Banks, where fog hindered visibility for both the air patrols and the boats of the 'Leuthen' wolfpack. Even so, Hepp’s U-238 was able to penetrate the escort screen and sank the 5,096-ton Norwegian freighter Skjelbred, 3,642-ton Norwegian freighter Oregon Express, and 7,134-ton British freighter Fort Jemseg. Kapitänleutnant Herbert Engel’s U-666 torpedoed Itchen, which sank, leaving just three survivors of her own crew and those of Polyanthus and St Croix she had rescued. Curio’s U-952 sank the 6,198-ton US freighter Steel Voyager and damaged the US freighter James Gordon Bennett. U-758 attacked, but had no hits confirmed and was itself damaged by a depth-charge attack.

Poor visibility, fuel shortages and exhaustion now afflicted the crews of both the U-boats and the escorts, but Dönitz, in the belief that his boats' attack had been very successful, ordered the 'Leuthen' wolfpack to end its efforts. However, while the claims by the various boats amounted to 12 escorts and nine merchant ships sunk, together with another two merchant ships damaged, the actual Allied losses were three escorts and six merchant ships sunk, together with one warship and one merchant ship damaged. The Germans had lost three U-boats destroyed and another three damaged sufficiently to be forced back to base.

Both the convoys continued to their destinations, the ONS.18 convoy reaching Halifax on 29 September and the ON.202 convoy reaching New York City on 1 October.

Believing the new tactics and weapons to be a great success, Dönitz continued the autumn U-boat offensive. The 'Leuthen' wolfpack was disbanded, 12 of its boats forming the core of the new 'Rossbach' wolfpack in which they were supplemented by nine newly arrived boats for an attack on the next set of eastbound convoys. The Admiralty was also encouraged by the result of the battle for despite the losses, which were serious, nine out of 10 merchant ships had arrived safely. The Admiralty was therefore confident that the escorts and their tactics and weapons would be able to meet the challenge of the U-boat arm’s new weapons and tactics.