17 January 1944 as WAO-124, USS Big Horn (AO-45/WAO-124/IX-207) was a Q-ship of the United States Navy named for the Bighorn River of Wyoming and Montana.

A disguised heavily armed merchantman, the decoy ship was intended to lure unsuspecting U-boats to the surface and sink them with gunfire.

While at Boston, Big Horn completed her disguise as a fleet oiler and was given extra watertight integrity – in case she was torpedoed – by the installation of thousands of sealed empty drums in her cargo tanks.

After two days on the degaussing range and in calibrating compasses and radio direction finders, Big Horn proceeded to Casco Bay for training under Commander, Destroyers, Atlantic Fleet.

This training period was followed by a shakedown cruise which was completed on 26 August 1942, at which date USS Big Horn put in again at the Navy Yard, Boston, for further alterations and repairs until 12 September.

As U-boats had been attacking bauxite ore cargo ships in the West Indies, the Q-ship sailed south to help defend the convoy routes there on 27 September.

1942 The first cruise of USS Big Horn began on 27 September 1942, when the ship proceeded from New York with a convoy bound for Guantánamo Bay, Cuba, taking a position which permitted the vessel to act as a straggler.

That same afternoon, three U-boats attacked the convoy, and at 15:20 in 11°00′N 61°10′W / 11.000°N 61.167°W / 11.000; -61.167, the British steamer SS Castle Harbour was hit on the starboard side by a torpedo and sank in less than two minutes.

Soon afterwards, lookouts on Big Horn sighted a U-boat moving at periscope depth on the port beam, but in such a position that no action could be taken without damaging the United States troopship Mexico or the Egyptian ship Raz El Farog.

At 16:27, lookouts on Big Horn again sighted a periscope and conning tower, on the port side, and her four-inch (100 mm) gun was trained in that direction just as a submarine chaser crossed through the line of fire and dropped five depth charges.

Convoy UGS-7A sailed on the morning of 14 April 1943, and the special Task Group joined up off New York and continued in company until 08:00 on 21 April, when the Group left the convoy and dropped astern 25 nautical miles (46 kilometres), proceeding as straggler-with-escorts, although the escorts remained far enough astern so that they would not be visible to an enemy submarine sighting Big Horn.

Early that morning, Big Horn had made radar contact with a suspected U-boat at a range of about six miles (9.7 km) and sent the two PCs to investigate.

At 12:35, Big Horn got a sound contact and delivered a Hedgehog attack just after sighting a periscope on the starboard bow at 12:42, followed by a heavy swirl as the U-boat dove.

About five of the Hedgehog projectiles (which detonate only on contact) exploded about 12 seconds after they entered the water, and Big Horn continued in to drop depth charges.

As none of the vessels in the Group were able to establish contact during the next 44 hours, it was presumed that one submarine had been destroyed; that the other U-boat which had been sighted by the PC-618 had moved out of the area.

After training exercises in the New London area with a friendly sub from 29 October through 10 November, Big Horn made one more uneventful cruise in company with PC-617 and PC-618.

Big Horn then steamed back to New London on 30 November, whence she conducted training in Long Island Sound, before departing on a third "decoy" cruise on 19 December.

Although she operated near a suspected U-boat concentration in the waters off Bermuda, the Q-ship and her two subchasers had no contacts and returned to New York empty-handed on 30 December.

After loading 84,000 barrels (13,400 m³) of oil at Aruba, Netherlands West Indies, on the 18th, she passed through the Panama Canal on 21 March and reported to the Service Force, Pacific Fleet, that same day.

Steaming west, Big Horn stopped at Pearl Harbor in early April before sailing on to the Marshall Islands, where she anchored at Ulithi on 1 May.

Assigned to Service Squadron 10 (ServRon 10), the shuttle tanker carried oil to Kossol Roads and Peleliu in the Western Carolines in early May before moving on to Tacloban in the Philippines later in the month.

Departing Okinawa on 29 September, Big Horn steamed to Japan, where she was assigned duty as a station tanker at Nagoya on 3 October.