During the final months of World War I, O-9 operated on coastal patrol and protected the Atlantic coast from U-boats.

Proceeding to Coco Solo, Panama Canal Zone, in 1924, the boat was reclassified to a second line submarine during her year there.

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 Tambor-class submarines were already nearing completion and 73 Gato class boats had already been ordered when O-9 was recommissioned at Philadelphia, on 14 April 1941, and went to New London on 31 May.

Upon reaching their designated training area the following day, some 15 mi (24 km) off Portsmouth, New Hampshire, O-6 made the first dive, followed by O-10.

On 22 June, Secretary of the Navy Frank Knox conducted memorial services for the 33 officers and men lost on the boat.

[2] More recently, and following Lovecraft's own pretense of presenting a pseudo-historical tale, Kenneth Hite and Kennon Bauman have depicted the submarine as being O-9 in their book The Cthulhu Wars.

[4] Another real-life vessel mentioned by Hite and Bauman is the USCGC General Greene (WPC-140; later WSC-140 and WMEC-140), which served in the United States Coast Guard from 1927 until decommissioning on 15 November 1968; this cutter was sold in 1976.