The seizure of Ujelang

On 20 April 1944, Onslow was given the task of seizing Ujelang, a small island in the Marshall Islands occupied by Japanese troops. Embarking troops of the United States Army's 111th Infantry Regiment she rendezvoused with minesweeper USS YMS–91 on 21 April 1944. On the morning of 22 April 1944, 160 soldiers were put ashore to capture the island. They met resistance at the center of the island from 18 entrenched Japanese soldiers, who were killed, with no American casualties. During this time, Onslow came under attack by a twin-engined Mitsubishi G4M "Betty" bomber, which she drove off with 5-inch (127 mm) antiaircraft fire.

Operations at Kwajalein and Eniwetok

Onslow returned to Kwajalein on 25 April 1944 and spent the next six weeks there engaged in seaplane tending duties. She then was sent to Eniwetok, arriving there on 10 June 1944.

Supporting landings on Saipan

On 15 June 1944, in company with destroyer USS Downes (DD-375), Onslow escorted seaplane tender USS Pocomoke (AV-9) to Saipan. While on this passage she rescued a downed fighter pilot who was adrift in a raft, 25 nautical miles (46 km) off Saipan.

On 18 June 1944, three days after the initial landings on Saipan, Onslow took up duties tending the seaplanes of Patrol Squadron 16 (VP-16).

On the morning of 10 September 1944, two Japanese swam from Saipan and succeeded in climbing aboard one of the moored seaplanes undetected. They proceeded to blow the plane up with hand grenades, and Onslow dispatched a boat to assist the plane's crew. The boat was driven back under fire, but two additional boats joined in and they were then able to reach the plane and kill the boarders. However, gunfire set the plane's fuel ablaze, destroying it, although its crew was saved.

Operations in the Palau Islands

Onslow and seaplane tenders USS Pocomoke, USS Chandeleur (AV-10), USS Yakutat (AVP-32), and USS Mackinac (AVP-13) departed Saipan on 12 September 1944 to participate in the invasion of the Palau Islands. At noon on 16 September 1944 the formation entered Naval Base Kossol Passage. The five ships tended seaplanes there under adverse conditions that included floating mines, dangerous coral heads, and rough water, not to mention the presence of enemy airplanes and submarines.

On 13 November 1944, Onslow sped out to recover a Martin PBM Mariner flying boat down at sea with engine failure and towed it 275 nautical miles (509 km) to Ulithi Atoll. She then returned to Kossol Passage.

On 12 January 1945, three Japanese midget submarines entered the harbor, and the ships spent the day dodging coral heads trying to ferret out the elusive little submarines. One of the midget submarines was sunk; the other two escaped.

Operations at Saipan

On 17 January 1945, Onslow moved to Ulithi Atoll, and on 19 January 1945 she steamed for Guam. She then spent two months at Saipan beginning on 20 January 1945, providing services to CASU 48 bowser boats and the crews of two crash boats.

Supporting the invasion of Okinawa

On 23 March 1945, Onslow got underway for the invasion of Okinawa. Along with seaplane tenders USS Yakutat, USS Shelikof (AVP-52), and USS Bering Strait (AVP-34), she escorted seaplane tenders USS Hamlin (AV-15), USS St. George (AV-16), and USS Chandeleur from Saipan to Kerama Retto, arriving there on 28 March 1945. Upon anchoring, the ships began to lay out a seadrome and by 29 March 1945, were operating on a regular schedule with 60 PBM Mariner seaplanes.

Onslow's work at Kerama Retto was more difficult than ever before. Many of the seaplanes were damaged by the Japanese or by heavy seas. Because of their frequency, it became necessary for the seaplane tenders to ignore Japanese air raids at times. On one occasion Onslow assisted in shooting down a Japanese fighter which was making a dive on her.

Beginning about 1 June 1945, many of Japanese troops isolated on the small islands off Okinawa began coming to the beaches to surrender. When they did, Onslow notified the proper commands and often furnished shallow-draft boats to assist the U.S. Army in picking up the surrendering Japanese.

On 6 August 1945, Onslow received orders to escort Chandeleur to Eniwetok via Saipan. She transferred all patrol squadron personnel and equipment to seaplane tender USS Duxbury Bay (AVP-38) and departed Okinawa. Hostilities with Japan ceased and World War II came to an end on 15 August 1945, the same day she entered port at Eniwetok.

Honors and awards

Onslow earned four battle stars for her service in World War II, for:

• Marshall Islands operation, March 1944-April 1944
• Marianas operation: Capture and occupation of Saipan, 14 June 1944 – 27 August 1944
• Western Caroline Islands operation: Capture and occupation of southern Palau Islands, 16 September 1944 – 13 November 1944
• Okinawa Gunto operation: Assault and occupation of Okinawa Gunto, 28 March 1945 – 6 August 1945