'O' (i) was the Japanese occupation of Kavieng on the Australian-mandated island of New Ireland in parallel with the 'R' occupation of Rabaul on New Britain (23 January 1942).

New Ireland is the second largest island of the Bismarck archipelago to the north of New Guinea, and is a long but narrow body of land angled primarily along a north-west/south-east axis. About 200 miles (320 km) long and between 8 and 20 miles (13 and 32 km) wide, with an area of 2,859 sq miles (7404 km²), the island is located to the north-east of New Britain and to the north-west of the Solomon islands group, and shares the thickly jungled terrain of both. There was a good but undeveloped anchorage at Kavieng on the north-western tip of the island in 1941, but this tended to be overshadowed by the superb anchorage at Rabaul on the north-eastern tip of New Britain. As well as being covered with jungle, New Ireland is rugged, and its highest point is Mt Taron, with a height of 7,805 ft (2379 m). The mountains drop steeply into the ocean along the south-west coast, but descend more gradually to the north-east, where there is a narrow coastal plain. In 1941 the indigenous population numbered about 54,000 persons speaking more than 45 different dialects, and at the end of this same year the island also had about 200 European residents.

The only Australian force available for the defence of New Ireland was part of the 1^st Independent Company located at Kavieng.

On 20 January the light cruisers Tatsuta and Tenryu arrived off Kavieng with three troop transports (9,310-ton Kinryu Maru, 8,469-ton Goyu Maru and 7,613-ton Azumasan Maru) escorted by the destroyers Kikuzuki, Uzuki and Yuzuki. The heavy cruisers Aoba, Furutaka, Kako and Kinugasa provided cover, together with the seaplane tender Chitose. More distant cover was provided by the fleet carriers Akagi, Kaga, Shokaku and Zuikaku, battleships Hiei and Kirishima, and heavy cruisers Chikuma and Tone. Two oilers, the 8,691-ton Hoyo Maru and 10,020-ton Shinkoku Maru, remained to the north of New Ireland.

The Japanese launched a major attack on Kavieng during 21 January 1942 using aircraft from the seaplane tender Kiyokawa Maru, and the small Australian defence unit withdrew into the jungle after cratering the runway of the airstrip, which it had only just completed. Some 500 men of two companies of the 2^nd Maizuru Special Naval Landing Force from Guam in the Mariana islands group occupied Kavieng on 23 January, and seven days later the Australian unit left New Ireland aboard its station ship. This was damaged by a Japanese air attack and escorted to Rabaul where the troops and crew surrendered. The estimated 200 Europeans remaining on the island were interned.

The Japanese later established the 14^th Base Force and elements of the 5^th Kure Special Naval Landing Force and 5^th Sasebo Special Naval Landing Force at Kavieng, whose harbour was developed as a secondary fleet base to support Rabaul. Detachments of the special naval landing forces played a more active role when they were committed to the ill-fated 'Re' attack of August 1942 on Milne Bay at the eastern end of New Guinea.

Elements of Vice Admiral Jinichi Kusaka’s 11^th Air Fleet were stationed on New Ireland with airfields at Kavieng, Panapai just to the south-east of Kavieng, and Namatanai and Borpop on the lower part of the north-east coast. The naval commander at Kavieng was Rear Admiral Ryukichi Tamura.

By July 1942 the Allies had decided to seize New Ireland, scheduling the invasion for April 1944, and planned to establish a minor fleet base and six airfields from which to exert greater pressure on Rabaul, and later to support operations into the Marshall and Caroline island groups. The Allies periodically bombed Kavieng, and this increased in frequency and intensity as the operations associated with the 'Elkton' and 'Cartwheel' plans closed the ring round Rabaul. The Japanese strengthened Kavieng to help protect Rabaul in the period between late 1943 and early 1944, and at the same time the Allied air attacks increased. The 230^th Regiment of Lieutenant General Tadayoshi Sano’s 38^th Division arrived from Rabaul during October 1943, and was followed by the 1^st Independent Mixed Regiment from Japan in the period between late 1943 and early 1944: both regiments came under the command of Major General Takeo Ito, commander of the 38^th Division's infantry group. The two regiments were formed into the 40^th Independent Mixed Brigade at Namatanai in May 1944 and were directly subordinate to General Hitoshi Imamura’s 8^th Area Army at Rabaul. At this time the Japanese strength on New Ireland was in the order of 10,000 army and navy personnel.

The Japanese attempted to reinforce the garrison in December 1943, using the super-battleship Yamato as a troop ship, but the US submarine Skate scored a torpedo hit on Yamato on 25 December 1943, the resulting damage being sufficient to force the cancellation of this reinforcement attempt.

Kavieng was came under air attack on the same day when the carriers of Rear Admiral Frederick C. Sherman’s Carrier Division 1, operating as Task Group 50.2, raided the island with 86 aircraft and found the anchorage almost deserted. A weak counterattack that night by four Mitsubishi G4M 'Betty' medium bombers and seven Nakajima B5N 'Kate' torpedo/level bombers failed to inflict any damage. There were further raids in the next few weeks, including one on 1 January 1944 against a convoy returning from Rabaul to Truk that did little damage but drew off Japanese air strength during the 'Dexterity' landings at Saidor.

The Allied assault on Kavieng, planned for implementation in April 1944, was 'Lockjaw' by Major General Allen H. Turnage’s 3^rd Marine Division and Major General Rapp Brush’s 40^th Division, but this was cancelled on 12 March. This cancellation was the result of the decision that the assault would not be worth the casualties as both Rabaul and Kavieng had been effectively neutralised by air and naval power. The islands of which the Allies had taken control around Rabaul also served to isolate New Ireland, and thereafter Japanese forces sat out the rest of the war in increasingly dire straits as a result of malnutrition and disease.

On 19 September 1945, Lieutenant General Kenneth W. Eather, commanding the Australian 11^th Division, accepted the Japanese surrender.