As part of Vice Admiral R. K. Turner's Northern Attack Force, she departed Pearl Harbor 29 May; touched at Eniwetok; and carrying troops of the 4th Marine Division, arrived off Saipan in the early hours of 15 June. She debarked her troops in the initial assault waves, then discharged cargo as bitter fighting raged on shore. After embarking casualties and enemy prisoners, she departed 17 June and cruised northeast of Saipan while Vice Admiral Marc Mitscher's Fast Carrier Task Force defeated Admiral Ozawa's Mobile Fleet in the Battle of the Philippine Sea, the greatest carrier battle of the war. Following the resounding American victory, "James O'Hara" returned to Saipan 23 June; completed unloading cargo; and departed 24 June for Eniwetok and Pearl Harbor.