up to . O'Briens main battery consisted of four /50 caliber Mark 9 guns, with each gun weighing in excess of . The guns fired armor-piercing projectiles at . At an elevation of 20°, the guns had a range of . O'Brien was also equipped with four twin 21 inch (533 mm) torpedo tubes. The General Board of the United States Navy had called for two anti-aircraft guns for the O'Brien-class ships, as well as provisions for laying up to 36 floating mines. From sources, it is unclear if these recommendations were followed for O'Brien or any of the other ships of the class. Pre-World War I O'Brien was commissioned into the United States Navy on 22 May 1915 under the command of Lieutenant Commander C. E. Courtney, after which she conducted her shakedown cruise between Newport, Rhode Island, and Hampton Roads, Virginia. In fleet exercises off New York in November, O'Brien collided with the destroyer , in a minor incident that carried away part of Draytons topmast and wireless gear. In December, she was assigned to the 5th Division, Torpedo Flotilla, Atlantic Fleet. From early 1916-spring of 1917, she operated with the Fleet along the East Coast and in Cuban waters. At 05:30 on 8 October 1916, wireless reports came in of a German submarine stopping ships near the Lightship Nantucket, off the eastern end of Long Island. After an SOS from the British steamer was received at about 12:30, Rear Admiral Albert Gleaves ordered O'Brien and other destroyers at Newport to attend to survivors. The American destroyers arrived on the scene at about 17:00 when the U-boat, under the command of Kapitänleutnant Hans Rose, was in the process of stopping the Holland-America Line cargo ship . Shortly after, U-53 stopped the British passenger ship . As Rose had done with three other ships U-53 had sunk earlier in the day, he gave passengers and crew aboard Blommersdijk and Stephano adequate time to abandon the ships before sinking the pair. In total, 226 survivors from U-53s five victims were rescued by the destroyer flotilla. In February 1917, one of O'Briens gun crews hit a target at eight times in eight attempts with one of the destroyer's guns, a feat which earned the crew and O'Brien recognition in The Independent, a weekly newsmagazine published in Boston. World War I Returning from winter maneuvers off Cuba in March 1917, the ship