After the liberation of Dokkum, Kollum and Dokkumer Nieuwe Zijlen, the attention of the Canadian troops turned to the port of Oostmahorn. It was the gateway to the island of Schiermonnikoog, which was part of the German defence line, the Atlantic Wall. This bulwark ran along the coasts of France to Norway. During the war, and extensive occupying force was stationed on the Wadden Island.  

The village of Oostmahorn was "relieved" by the Dutch Domestic Armed Forces (NBS) on Saturday, 14 April, after the German Grenzschutz (border control) left in silence for Schiermonnikoog. Meanwhile, a group of more than a hundred fleeing SS and SD men and a dozen women crossed the Lauwerszee from Groningen near Zoutkamp with three boats to the island. A number of them were high-ranking SD members of the infamous Scholtenhuis in Groningen. When the NBS saw them sailing past from the port of Oostmahorn, shots were fired back and forth. 

The German occupying forces on Schiermonnikoog then wanted to take revenge. The village and the port of Oostmahorn were attacked from the island for two hours on Monday, 16 April. A convoy of three Dutch boats left for the port with the aim of landing there. Meanwhile, the help of the B-Squadron of the Royal Canadian Dragoons (1st Armoured Carrier Regiment) in Dokkum had been called in. The attack was narrowly repelled, but two Canadian soldiers from a reconnaissance group were killed. A defence line of foxholes and armoured vehicles was erected around the village. 

Further reinforcements were called in that day from Leeuwarden, where C Company of the North Nova Scotia Highlanders were sent to the coastal strip to patrol for German landing attempts.  
 
After a brief but heavy shelling from the Canadians as a warning, a mediator was sent to the island the next day. The highest-ranking SD officer refused to surrender. They were waiting for an escape with a Schnell boat that was to take them to the German Wadden island of Borkum on 3 May. That turned out to be in vain, but in the meantime they kept the island occupied, even after the general capitulation of the whole of the Netherlands on 5 May. 

It was not until 11 June that two boats, the MS Waddenzee and the MS Brakzand, transported the last occupiers of Schiermonnikoog to the port of Oostmahorn. A column of army trucks packed with prisoners of war left the port under the guard of the Allies. From there they went to the POW camp in Beerta (Groningen).