USS O-4 (SS-65) was an O-class submarine of the United States Navy. Her keel was laid down on 4 December 1916 by the Fore River Shipbuilding Company in Quincy, Massachusetts. She was launched on 20 October 1917, and commissioned on 29 May 1918 with Lieutenant Robert H. English (1888–1943), in command.
O-4 operated out of Philadelphia, Pennsylvania, during World War I and patrolled the U.S. Atlantic coast from Cape Cod to Key West, Florida.
On 24 July 1918, a British steamer mistook O-4 and O-6 (SS-67) for German U-boats and fired on the submarines. Although O-4 received six hits from the steamer, she suffered no major damage. In November, she joined the 20-submarine contingent that departed Newport, Rhode Island, on 3 November for European waters, however, hostilities ceased before the boats had reached the Azores.
O-4 then sailed to New London, Connecticut, to train students at the Submarine School there. Reclassified to a second line submarine on 26 July 1924, and reverting to a first liner on 6 June 1928, she trained Submarine School students at New London until 1931, with the exception of a brief tour at Coco Solo. O-4 decommissioned on 3 June 1931.
The approach of World War II saw the recall of O-4 to active service. She recommissioned on 29 January 1941 and trained students at the Submarine School until war's end. After the war, she steamed to Portsmouth, New Hampshire, to decommission there on 20 September 1945. She was struck from the Naval Vessel Register on 11 October 1945, and scrapped on 1 February 1946.