10 May - 12 May 1940: O13 patrols off the Dutch coast. She is attacked by German planes several times.
May 1940: O13, escorted by the minesweeper Jan van Gelder, sails to the U.K.
12 May 1940: O13 arrives in Portsmouth (England).
15 May - June 1940: O13 is based at Portsmouth and is under British operational control.
??: O13 is equipped with a loop-aerial.
29 May - 1 June: O13 patrols the English Channel (evacuation of Dunkirk and Bordeaux). One night, off the French coast, she encounters a German U-boat.
June 1940: O13 is based at Portland (England) under British operational control.
5 June - 7 June: O13 sails to Dundee (Scotland).
6 or 7 June: O13 arrives at the King George Wharf, Dundee.
11 June - ? 1940: O13 is attached to the 9th Flotilla in Dundee.
12 June 1940: O13 departs for a patrol near the entrance of the Skagerrak,
position: 57°N-05°E. The O13 and all hands (34) are lost during the patrol.
There are three possible explanations for the loss of the O13:
1) 13 June 1940: A German mine strikes the O13 near the Great Fisher Bank. O13 crosses the German minefield (named 16B or 168) at approximately 56°55'N-03°40'E.
2) 16 June 1940: O13 is bombed by two Arado type planes from the German battleship Scharnhorst. The planes reported a position 400 m north of the patrol area of O13. It is more likely the planes attacked the British submarine Porpoise.
3) 20 June 1940, 00:25 hrs: O13 is accidentally rammed by the Polish submarine O.R.P. Wilk, whose location was reported to be 56°50'N-03°37'E, only 16m from the estimated position of O13. According to details described by Wilk's XO Cdr. Romanowski in his memoirs it is unlikely the Polish submarine rammed the O13.
The first option seems to be the most likely.