'EO' was a British naval sortie by a major element of Admiral Sir John Tovey’s Home Fleet to undertake a carrierborne air attack on German shipping in and off the port of Tromsø in German-occupied Norway (20/23 February 1942).

As a result of 'Anklet' and 'Archery', Adolf Hitler, who was already concerned about the possibility (or even probability) of a British invasion of Norway, had become still more anxious. To serve as a deterrent to any such invasion or other smaller British landings, Grossadmiral Erich Raeder, the commander-in-chief of the German navy, proposed that the battleship Tirpitz be sent to Norway where, moreover, she would be ideally positioned to interdict Allied convoys on their passage to and from ports in northern Russia. After some hesitation, Hitler agreed to Tirpitz's deployment to Norway, and late on 14 January 1942 the battleship, escorted by the destroyers Richard Beitzen, Paul Jacobi, Bruno Heinemann and Z-29, departed Wilhelmshaven in northern Germany for Trondheim on the western coast of occupied Norway, arriving in the Fottenfjord late on 16 January.

At 07.30 on the following day, Tovey received information that Tirptiz might be at sea, and inconclusive evidence suggested that the Germans could possibly be undertaking an operation or movement other than a breakout into the Atlantic. Tovey nonetheless had to make the dispositions necessary to prevent any such possibility, and at 08.00 ordered all of the Home Fleet ships at Scapa Flow in the Orkney islands group to raise steam. At 16.00 the battleships King George V (flagship) and Rodney, the fleet carrier Victorious, the heavy cruiser Suffolk, the light cruisers Nigeria, Kenya and Sheffield, and the destroyers Ashanti, Bedouin, Echo, Escapade, Eskimo, Faulknor, Inglefield, Intrepid, Marne and Panther departed Scapa Flow in the Orkney islands group for the Hvalfjörður in south-western Iceland, which the ships reached on 19 February.

At 20.00 on 20 February the ships departed the Hvalfjörður on a course to the south-west.

(At 14.28 on 22 January Tovey had signalled the Admiralty that every endeavour should be made to damage Tirpitz in harbour by bombing or sabotage, and at the same time suggested that the Arctic convoys should continue singly. One day later Tirpitz had been located and photographed by a Supermarine Spitfire single-engined aeroplane of the RAF’s No. 1 Photographic Reconnaissance Unit as she lay at anchor at the head of the Åsfjord, 15 miles (24 km) to the east of Trondheim, in the Fottenfjord. This fjord is a mere 1,315 yards (1200 m) wide with steep cliffs on three sides, and Tirpitz was berthed toward the fjord’s northern shore below a steep cliff. The battleship was well camouflaged and protected from attack by anti-submarine nets and protective booms in the water as well as anti-aircraft and searchlight positions on the surrounding cliffs and islands. On 31 January the first attempt on Tirpitz in Norway took place as 'Oiled'.)

On 24 January 1942 the Home Fleet’s ships returned to the Hvalfjörður, and remained there until 18 February to cover the Denmark Strait and Færoes-Iceland gap.

At 08.00 on 19 February the Home Fleet detachment moved from the Hvalfjördur to the Seyðisfjörður in south-eastern Iceland, which it reached at 08.30 on the following day, the destroyers immediately moving deeper into the fjord to refuel. At 16.00 the detachment reassembled off the Seyðisfjörður and set course for Tromsø to carry out the 'EO' carrierborne air attack.

At 22.00 on 20 February German 'pocket battleship' Admiral Scheer and heavy cruiser Prinz Eugen departed Brunsbüttel in northern Germany under the command of Vizeadmiral Otto Ciliax in the 'Sportpalast' (i) redeployment to Norway. From 'Ultra' intelligence the Admiralty’s Operational Intelligence Centre knew of the movement and requested RAF Coastal Command to undertake additional reconnaissance patrols, and at 12.10 on 21 February a Lockheed Hudson GR.Mk V twin-engined aeroplane, flying from North Coates, sighted and reported Admiral Scheer and Prinz Eugen, escorted by five destroyers and two torpedo boat, off Jutland and on a course to the north. The Admiralty assumed that this force was making for Trondheim, and on receipt of the information Tovey at 14.00 immediately abandoned 'EO' and altered course to the south in the hope of effecting an interception with the battleship King George V, the carrier Victorious, the heavy cruiser Berwick, and the destroyers Ashanti, Bedouin, Eskimo, Icarus, Onslow, Punjabi and Tartar.

However, at 13.00 on 21 February the B-Dienst signals interception and analysis detachment on Prinz Eugen had decoded the Hudson’s sighting report, and Ciliax had immediately ordered a reversal of his ships' course to return to Germany. At 17.30 Ciliax was ordered to revert to his original course once again by Generaladmiral Rolf Carls’s Marinegruppenkommando 'Nord', and Ciliax complied at 19.40 and at the same time detached the two torpedo boats. At 08.15 on 22 February the German force was off Karmsund, and at 12.00 anchored in the Grimstadtfjord to the south of Bergen. At 20.00 Ciliax’s ships departed the Grimstadtfjord for Trondheim, at about 03.30 on 23 February were off Stadtlandet, and at 07.00 the British submarine Trident torpedoed Prinz Eugen, causing severe damage to the heavy cruiser’s stern.

Meanwhile, on 22 Febuary Victorious, Berwick, Ashanti, Bedouin, Eskimo and Icarus had been detached to steam at high speed in order to reach a point some 115 miles (185 km) off Stadtlandet at 01.00 on 23 February, the rest of Tovey’s ships following at a slower speed to provide cover.

At 01.30 and in very bad weather, Victorious managed to launch 10 Fairey Albacore single-engined attack aircraft of No. 832 Squadron, and 30 minutes later seven more Albacore aircraft, this time of No. 817 Squadron. Only one of the first-wave aircraft and two of the second-wave machines were equipped with surface-search radar. After launching the two waves of aircraft, Victorious and her escorts immediately shaped course to rejoin the rest of Tovey’s force and then steam to Scapa Flow.

As a result of the weather conditions, the attack force achieved nothing, though it lost three aircraft, and then flew toward the RAF base at Sumburgh in the Shetland islands group.

At 22.30 on 23 February Tovey’s ships arrived back at Scapa Flow.