'OG' was the designation of Allied convoys (together with a numerical suffix) plying the route from the UK (later only Liverpool) south to Gibraltar, and as such reciprocals of the 'HG' series (October 1939/October 1943).

These 'Outbound to Gibraltar' convoys were halted temporarily in August 1942 and restarted in May 1943, from July 1943 sailing in company with the KMS convoys until the series ended.

The 'OG' series of mercantile convoy began in October 1939 and represented an amalgamation of the 'OA' series from Southend and the 'OB' series from Liverpool. The two convoys of the same number would meet some 865 miles (1390 km) to the west of Land’s End to become the 'OG' series and sail onward to Gibraltar. This practice was continued until July 1940 when the convoys of the 'OG' series sailed directly from Liverpool. There were 117 such convoys, which ended when the KMS series was inaugurated.

The first of the OG series of convoys was OG.1, which formed at sea on 2 October 1939 and reached Gibraltar on 8 October, and comprised 38 merchant vessels escorted by six warships (light cruiser Ceres and destroyers Ilex, Imogen, Imperial, Isis and Velox).

The last convoy of the series was OG.89 which departed Milford Haven on 30 August 1942 and reached Gibraltar on 14 September, and comprised 21 merchant vessels escorted by nine warships (sloops Black Swan and Fowey, corvettes Carnation, Coreopsis, Mallow and Myosotis, anti-submarine trawlers Birdlip and Southern Wave, and armed boarding vessel Goodwin).

Two of the 'OG' convoys involved in major battles with U-boats were OG.69 and OG.71.

The battle round the OG.69 convoy came at the time, between 18 and 30 July 1941, when joint German and Italian submarine operations were launched against the British convoys to and from Gibraltar. On 18 July Axis agents in neutral Spain observed the departure of the HG.67 convoy, and the Italian submarines Alessandro Malaspina, Morosini, Luigi Torelli, Alpino Bagnolini and Barbarigo were concentrated for an attack, but the convoy evaded this Italian effort. After Luigi Torelli had sunk a single 8,913-ton tanker, Barbarigo sighted the HG.67 convoy on 22 July but soon lost contact, with the result that the Italian boats and the German U-93, U-94, U-124 and U-203, which had been despatched to the area, were unable to find it. Alpino Bagnolini had briefly sighted a British homebound convoy on 19 July, and then attacked the OG.68 convoy on 23 July. The crew of the boat heard three detonations, but these were not torpedo hits. Barbarigo sank two independent sailers totalling 13,407 tons on 25/26 July.

On 24 July the B-Dienst radio intercept and decryption service was able to fix the positions of the OG.69 and SL.80 convoys, as a result of which U-79, U-126, U-331, U-68, U-561, U-562, U-564 (only briefly) and U-203 were directed to concentrate against OG.69, and U-431, U-565, U-401, U-74, U-94 and U-97 against SL.80.

On 25 July the current positions of SL.80 was reported once and that of OG.69 twice by Focke-Wulf Fw 200 long-range aircraft of of Major Adolf Koch’s I/Kampfgeschwader 40, and as many as 15 U-boats received the bearings, but on 26 July contact with SL.80 could not be re-established and the effort to intercept was terminated.

On the same day, off Northern Ireland, Oberleutnant Philipp Schüler’s U-141 attacked the OS.1 outbound convoy, which was escorted by Commander D. MacIntyre’s British 5^th Escort Group (led by the destroyer Walker) and sank the 5,106-ton British Botwey, and torpedoed and damaged the 5,133-ton British Atlantic City. The U-boat was then pursued for 20 hours with depth charges.

Fw 200 aircraft again established contact twice with the OG.69 convoy and, with the aid of bearings, brought U-68 into position during the afternoon. In addition to the seven German boats, the Italian Barbarigo and Pietro Calvi were also directed to the convoy, which comprised 28 merchant vessels with an overall total of 17 escort vessels in the form of the sloop Black Swan, corvettes Alisma, Begonia, Dianella, Jasmine, Kingcup, Larkspur, Pimpernel, Rhododendron, Sunflower and Free French Fleur de Lys, auxiliary anti-aircraft ship Goodwin, and armed trawlers Drangey, Lady Hogarth, Lady Shirley, Paynter and St Nectan.

The first U-boat contact with the convoy was achieved at 17.45 on 26 July, when U-68 located the convoy. Between 19.30 and 20.40 U-79, U-371 and U-561 also gained contact with the convoy. Kapitänleutnant Wolfgang Kaufmann’s U-79 delivered the first attack early in the morning of 27 July, sinking the 1,459-ton British Kellwyn. At much the same time U-331 also made contact with the convoy, but was almost immediately driven off by the escorts. U-126 and U-203, the latter commanded by Kapitänleutnant Rolf Mützelburg, arrived two hours later, and U-203 immediately sank the 2,475-ton British Hawkinge. At 03.30 U-79 and U-203 came under depth-charge attack from the escorts.

The U-boats shadowed the convoy during the daylight hours of 27 July and then moved to attack once more during the dark.

Two Fw 200 aircraft had again been keeping contact with the OG.69 convoy, and U-68, U-562 and U-126 approached, but only the last, under the command of Korvettenkapitän Ernst Bauer, was able to target victims, sinking the 1,335-ton British Erato and 1,304-ton Norwegian Inga I toward 24.00. During the night of 27/28 July Korvettenkapitän Robert Bartels’s U-561 sank the 1,884-ton British Wrotham.

On 28 July Fw 200 aircraft as well as U-68, U-79, U-561, U-331 and U-126 were all in intermittent contact, and during the evening U-203 sank the 1,330-ton British Lapland and 1,516-ton Swedish Norita.

An agent’s report about the departure of the HG.68 convoy from Gibraltar was received by the Germans at about this time. During the night of 28/29 July U-331 was driven away from the OG.69 convoy, but U-66, U-79, U-126, Pietro Calvi, Alpino Bagnolini and Barbarigo grouped on 29/30 July against both the OG.69 and HG.68 convoys, but then enjoyed no success.

The last attack had been made by U-203 in the evening of 28 July, and the last contact had been that of U-331 during the morning of 29 July. The OG.69 convoy had lost seven ships totalling 11,303 tons. The U-boats had fired 25 torpedoes and had been attacked eight times (six with depth charges and two with gun fire). Only U-562 was damaged, in a depth-charge attack, and returned to base for repairs.

After operating against the SL.81 convoy, from 10 August 1941 U-75, U-83, U-106, U-201, U-204, U-559 and U-564, together with the newly arrived U-552, assembled to the west of the North Channel. On 17 August these U-boats were directed toward the OG.71 convoy of 23 merchant vessels escorted by the British 5^th Escort Group comprising the sloop Leith, Free Norwegian destroyer Bath, corvettes Campanula, Hydrangea, Bluebell, Campion, Wallflower and Zinnia, and anti-submarine trawler Lord Nuffield, which were supplemented from 20 August by the destroyers Gurkha and Legion, and from the following day by the destroyer Boreas.

The convoy’s position had been reported by an Fw 200 aeroplane of the I/KG 40, and U-201 established contact late in the evening and, with a number of interruptions, maintained it until 19 August.

On 18 and 19 August, Junkers Ju 88 aircraft of Generalleutnant Martin Harlinghausen’s Fliegerführer 'Atlantik' command searched for the convoy, but managed to make only individual rather than formation contact. Guided by reports from U-201 and aircraft, Korvettenkapitän Adalbert Schnee’s U-201, Kapitänleutnant Walter Kell’s U-204 and Kapitänleutnant Hans Heidtmann’s U-559 were able to reach attack positions and fire torpedoes during the night: U-201 sank the 3,255-ton British Aguila carrying 1,288 tons of general cargo and 1,809-ton British Ciscar carrying stores, U-204 the destroyer Bath and U-559 the 1,584-ton British Alva.

In the evening U-106 assumed the contact-keeper role, but was driven off by the escorts. As noted above, on 20 August the destroyers Gurkha and Legion arrived to strengthen the escort, as did the destroyer Boreas on the following day.

The Germans lost contact until the afternoon of 21 August, when it was later regained by an Fw 200. Five more U-boats, in the form of U-106, U-108, U-201, U-552 and U-564, were directed to the convoy’s location, but none of the boats found the convoy.

In the light of numerous reports by aircraft, Oberleutnant Reinhard Suhren’s U-564 established contact during the afternoon of 22 August and brought up U-201 once again. During the night 22/23 August U-564 sank the 1,203-ton Irish Clonlara carrying coal and 484-ton British tug Empire Oak, and U-201 sank the 1,974-ton British Aldergrove carrying coal and the 787-ton British Stork carrying cased petrol. The 2,129-ton Norwegian Spind, carrying coal and already hit by U-564, was sunk by Oberleutnant Erich Topp’s U-552. In the morning of 23 August U-564 sank the corvette Zinnia from the convoy’s escort.

The overall result was that two escorts and nine ships totalling 13,225 tons had been sunk.