The "Olav Tryggvason" was at Horten for minor engine repairs, manned only by a skeleton crew, when the Germans invaded on 9 April 1940. Along with the minesweeper , she defended the harbour against the German torpedo boats "Albatros" and "Kondor", and two 120-ton "räumboot" minesweepers; the "R 17" and the "R 27".