In 1927 a protruding Telefunken radiopeiler was installed on the O 9 that allowed the boat to receive long wave radio frequencies underwater.

[2] In February 1943 the O 9 was equipped with a fire control system that was developed by its commander Drijfhout van Hooff and manufactured by Cambridge Instrument Company Ltd.[3] The submarine was ordered on 30 August 1921 and laid down in Flushing at the shipyard of Koninklijke Maatschappij De Schelde on 1 December 1923 or 23 September 1922.

[4] 21 June 1926, O 9, together with O 11, Maarten Harpertszoon Tromp, Jacob van Heemskerck, Z 7 and Z 8, sailed from Den Helder to the Baltic Sea to visit the ports of Kiel, Göteborg and Trondheim.

The next year on 30 July 1930 O 9, O 10, Jacob van Heemskerck and Witte de With visited Antwerp.

From August 1940 to March 1944 O 9 was attached to the 7th Training Flotilla in Rothesay and used as an ASDIC piggy boat.