During WWI, it was pressed into service as a troop carrier, armed with several 6 inch guns.
From the titanic-whitestarships web site:
"Olympic’s most notable achievement during the war was the ramming and sinking of of the German submarine U-103 on May 12, 1918. Olympic was the only merchant ship to sink an enemy warship during the war. Korvettenkapitän Claus Rücker, commander of the 9-month old U-103, had unsuccessfully fired two torpedo's at Olympic just missing her port bow. In what some historians consider a foolish maneuver, Olympic turned around and rammed the u-boat sinking her and killing the crew. It is believed that Olympic could have easily outrun the u-boat on a zigzag course and not have risked the lives of the thousands on board.The Olympic was decommissioned and sold for scrap in 1935 for about $500,000. Her luxurious fittings were removed and sold. Many of these items can be found today in various hotels, pubs and restaurants around the world.
Refitted and converted to oil fuel, Olympic returned to service in July of 1920, . With Britannic's sinking during the war, Olympic became the last remaining ship of the original Olympic Class trio of liners envisioned by WSL chairman Bruce Ismay. She was a favorite among the passengers in her own right as well as for the fact that she was about identical in appearance to lost sister Titanic. Interestingly, Olympic was the first White Star liner to sell out passage booking after Titanic sank, until Majestic ( "The Magic Stick") came along."
The Britannic still lies under 400 ft. of water off the Greek island of Kea. In 2008, it was proposed to turn the site into an underwater museum for tourists.