During the final months of World War I, O-9 operated on coastal patrol and protected the Atlantic coast from U-boats. She departed Newport, Rhode Island, on 2 November 1918 for Britain, in order to conduct her first war patrol. However, the end of the war came before O-9 reached Europe. 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, PanamaCanal 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. Remaining on the Naval Vessel Register, O-9 was recalled to training service as American involvement in World War II became more inevitable. The 12 s were already nearing completion and 73 boats had already been ordered when O-9 was recommissioned at Philadelphia, on 14 April 1941, and went to New London on 31 May. In all, eight of the original ten O-boats were recommissioned to serve as training submarines in the Second World War. O-9, in particular, required extensive work, and still suffered mechanical problems even after being returned to service. On the morning of19 June 1941, O-9 and two of her sisters, and, left as a group from the submarine base in New London, for the submarine test depth diving area east of the Isles of Shoals. Upon reaching their designated training area the following day, some off Portsmouth, New Hampshire, O-6 made the first dive, followed by O-10. Finally, at 08:37, O-9 began her dive. At 10:32, O-9 had not returned to the surface. Rescue ships swung into action immediately. Sister ships O-6 and O-10, submarine, submarine rescue ship, and other ships searched for O-9. That evening, pieces of debris with markings from O-9 were recovered. In water deep, she was thought to be crushed, since her hull was only designed to withstand depths of. Divers went down from 13:00 on 21 June until 11:43 on 22 June. Divers could stay only a short time at the depth but nonetheless set endurance and depth records for salvage operations until those operations were cancelled, as they were considered too risky. Rescue operations were discontinued on 22 June. The boat and her 33 officers and men were declared lost as of 20 June. On 22 June, Secretary of the NavyFrank Knox conducted memorial services for the 33 officers and men lost on the boat.
Wreck
On 20 September 1997, based on several years of research by Glen M. Reem the O-9 was finally located. Salem, New Hampshire-based Klein Sonar Company provided a vessel and sonar equipment which were used to discover O-9s final resting place. Her hull has been crushed from just abaft the conning tower all the way to the stern, though the forward hull appeared intact. There are no plans to salvage O-9. Her exact location is secret and the area has been designated an official Naval burial ground. O-9 was struck from the Naval Vessel Register on 23 October 1941.
