Although the German naval attack on Oslo had been thwarted by the actions of Oscarsborg, the city was seized later that day by forces that were airlifted into Fornebu Airport. In light of the fall of the capital, and with news of German landings at the village of Son south of Drøbak, Colonel Eriksen decided that further fighting without adequate infantry support was in vain, and agreed to a ceasefire in the evening of 9 April. The fortress was surrendered intact on the morning of 10 April.
The garrison at the main battery and at Håøya were treated separately by the Germans from those captured from the mainland batteries, and were released a week after the battle. The soldiers and non-commissioned officers captured at the mainland batteries were released three days after the fortress\’ surrender, while the officers were held as prisoners of war at Fredriksten Fortress. The reserve officers were released on 15 May, while the full-time officers were transferred to Grini prison camp and released in late May 1940.