At 11.17 hours on 28 Feb 1942, U-156 began to shell the unescorted and unarmed Oregon (Master Ingvald C. Nilsen) which was steaming completely blacked out on a zigzag course at 9.5 knots about 130 miles north of Mona Passage. The U-boat had first spotted the tanker about eleven hours earlier and Hartenstein could only attack with the deck gun as he had no torpedoes left. The Germans opened fire from a distance of only 900 meters as the deck gun had a shortened barrel due to an earlier incident during the patrol. The first shell struck the starboard side in the quarters of the master and the second struck near the bridge, destroying the radio shack and killing the master and the men on watch on the bridge. The tanker apparently went out of control and suddenly turned hard to port at 6 knots, colliding slightly with the forecastle of U-156 before stopping. Most of the eight officers and 28 crewmen abandoned ship in one lifeboat in calm seas, leaving on the starboard side while the port side was taken under fire. Four others were thrown over dove overboard and found safety on a raft. The U-boat circled and shelled the ship for 75 minutes, in between only with the anti-aircraft guns as the Germans had to repair the deck gun during the engagement after the counterweights fell off and then fired the last available rounds for the deck gun and the 37mm AA gun into the waterline at point blank range. Altogether 58 rounds of 105mm, 304 rounds of 37mm and 101 rounds of 20mm ammunition were expended during the attack. Small fires broke out on the bridge, in one of the fuel tanks and on the stern and eventually a boiler exploded but the cargo never caught fire. The U-boat did not wait for the burning tanker to sink and left the area without questioning the survivors.

The Oregon was seen to sink by the stern by the survivors about four hours after the initial attack. The master, two officers and three crew members were lost. The 26 survivors in the lifeboat made landfall near Puerto Plata, Dominican Republic at noon on 4 March and 16 of them were eventually taken to Guantanamo. The remaining ten were transported to Baracoa on Cuba and later repatriated aboard the Honduran steam merchant Guardian to Charleston. The four survivors on the raft were picked up by Gulfpenn after five and a half days and landed at Philadelphia on 10 March. Some of the survivors received the false impression that two U-boats shelled the vessel and reported that they were machine gunned when they tried to lower the port lifeboats. Furthermore a man stated that a U-boat tried to run him down while swimming in the water and he could actually touch the hull when passing alongside. However, no evidence indicates that the survivors were deliberately targeted during the attack as all casualties occurred during the initial shelling of the bridge area.