She was commissioned into the German Imperial Navy on 18 May 1918 as SM UB-125.

[Note 1] UB-125 was surrendered 20 November 1918 in accordance with the requirements of the Armistice with Germany.

Handed over to Japan, she served as O-6 in the Imperial Japanese Navy until 1921 when she was broken up in Kure.

Like all Type UB III submarines, UB-125 carried 10 torpedoes and was armed with a 10.5 cm (4.13 in) deck gun.

UB-125 would carry a crew of up to 3 officer and 31 men and had a cruising range of 7,280 nautical miles (13,480 km; 8,380 mi).