'Ostmark' was a U-boat wolfpack operation in the North Atlantic against the SC.121 convoy (6/11 March 1943).

The wolfpack comprised U-190, U-229, U-439, U-447, U-530, U-618, U-633, U-641, U-642, U-665 and U-666, and for the loss of Oberleutnant Bernhard Müller’s U-633 sank five ships (21,965 tons) and damaged one 3,570-ton ship.

Grossadmiral Karl Dönitz, commander-in-chief of the German navy but still in day-to-day command of the U-boat arm via Konteradmiral Eberhard Godt, its chief of operations, created the 'Westmark' wolfpack of 17 U-boats and, to its east, the 'Ostmark' wolfpack initially comprising nine boats (U-190, U-229, U-439, U-447, U-530, U-618, U-641, U-642 and U-665) out of the 'Neuland' wolfpack. Their target was the SC.121 convoy of 59 ships sported by Commander Paul R. Heineman’s US Escort Group A3 (US Coast Guard cutter Spencer, destroyer Greer, Canadian corvettes Dauphin, Rosthern and Trillium, British corvette Dianthus, and rescue ship Melrose Abbey).

After U-405 had been driven off, U-230 and U-566 established contact with the convoy during the night of 6/7 March. Kapitänleutnant Paul Siegmann’s U-230 slipped in without alerting the escort to sink the 2,868-ton British Egyptian, and the 6,116-ton British Empire Impala, which had remained behind to recover survivors, was sunk in the morning by Kapitänleutnant Hans-Jürgen Zetzsche’s U-591.

On 7 March U-228, U-230, U-409, U-526, U-591 and U-634 managed to maintain contact in spite of a storm with snow and hail showers, but could not attack. In the morning of 8 March the wind fell but visibility was still poor: U-527 missed the convoy but Kapitänleutnant Hans Möglich’s U-526 probably sank the 3,921-ton British Guido, which had become separated. In the evening of 8 March Kapitänleutnant Max Wintermeyer’s U-190, Kapitänleutnant Herbert Uhlig’s U-527, Zetsche’s U-591 and Kapitänleutnant Herbert Brünning’s U-642 each sank a straggler: these were the 7,015-ton British Empire Lakeland, 5,242-ton British Fort Lamy (carrying a 291-ton tank landing craft), 5,879-ton Yugoslav Vojvoda Putnik and 2,125-ton British Leadgate.

On 9 March US ships, in the form of the Coast Guard cutters Bibb and Ingham and the naval destroyer Babbitt, arrived from Iceland to reinforce the escort. Consolidated Liberator long-range patrol bombers of the RAF’s No. 120 Squadron were now providing air escort and drove off U-566, which was the contact-keeper. Of the boats coming up by the fall of night, including U-229, U-230, U-332, U-405, U-409, U-641 and U-665, two were attacked from the air and four were depth-charged by Babbitt, Rosthern and others. An attack by U-229 failed. In the evening of 9 March Kapitänleutnant Kurt Lange’s U-530 sank the straggling 3,058-ton Swedish Milos. During the night U-409 and Korvettenkapitän Rolf-Heinrich Hopman’s U-405 made almost simultaneous attacks, the former damaging the 7,519-ton British tanker Bullmouth (later sunk by Korvettenkapitän Hans Stock’s U-659) and sinking the 6,373-ton British Silverwillow, and the latter sinking the 4,665-ton Norwegian Bonneville. A little later Oberleutnant Robert Schetelig’s U-229 sank the 4,946-ton British Nailsea Court and damaged the 3,670-ton British Coulmore.

On 10 March the wind strengthened once again to storm force and, in driving snow and hail, only U-229 and U-616, which vainly fired torpedo salvoes, and briefly U-523 and U-642 came into contact with Allied merchant vessels, all of them independently routed ships. In the afternoon the last contact-keeper, U-634, was driven off. The warships of the escort group were hindered by the partial malfunctioning of their radar and radio equipment as a result of storm damage. The escort was further reinforced that day by the arrival of the British corvettes Campion and Mallow.

Early on 11 March the Germans broke off the operation.