Marine Observation Group 1 was activated 15 May 1945 at Ewa, Territory of Hawaii. Its mission was to provide administrative control for the VMO squadrons. VMOs 1, 4, 5, and 8 were attached on 15 June, but came under operational control of V Amphibious Corps. The Group Headquarters relocated to Kahului, Maui, on 20 June 1945.
MOG-1, with subordinate squadrons VMO-2 and VMO-5, embarked aboard USS Thomas Jefferson on 26 Aug 1945 for occupation duty in Japan. MOG-1 operated from Sasebo, Japan from 23 September 1945, until 7 January 1945, when it was deactivated.