After the war, "O-9" continued in Naval service and trained submarine crews at the Submarine School at New London, Connecticut. Proceeding to Coco Solo, Panama Canal Zone, in 1924, the boat was reclassified to a second line submarine during her year there. Returning to operate at New London, "O-9" reverted to a first line submarine on 6 June 1928. Sailing up to Portsmouth, New Hampshire in January 1930, the submarine returned to New London in March; the following February, she sailed to Philadelphia, Pennsylvania, to decommission there on 25 June 1931.