Before the Soviet Union occupied Estonia during World War II, around 130 people, mainly Estonian Swedes, lived on the island.

[3] The island's population varied considerably with time, for example the plague epidemic of 1710 left according to popular stories only a few people alive.

In 1914, during World War I, the German light cruiser Magdeburg ran aground and sank near the northern tip of Osmussaar.

In 1942, during the Occupation of Estonia by Nazi Germany, the islanders could return to their homes, but the approaching Red Army forced them soon to leave permanently.

In late September 1944, the Soviet Union ordered 50 galeas and 50 motor boats to be delivered with Finnish crews to Osmussaar on 2 October 1944.

The galeas and motor boats were taken into the state service involuntarily, but the crews were offered voluntary pay of 250 Finnish marks per day.

Detachment Arho served the Soviet military logistics on Hiiumaa and Saaremaa establishing connections to Tallinn.

[6] From Osmussaar many vessels sailed to Rohuküla of Haapsalu to support the Moonsund Landing Operation on the occupied Hiiumaa and Saaremaa islands.

On 4 October the Finnish galeas shipped Soviet soldiers, horses and cannons to Heltermaa of Hiiumaa.

Nine of the fifty galeases were lost at sea due to rough water; additionally, seven were destroyed during the war.

On 16 October the German submarine, U-481, sank the galeases Endla, Dan and Maria with its gun fire and ramming them.

The vessels of the detachment Arho were returned to Finland to Hanko and 2 November 1944, the headquarters of the Finnish navy ordered the liquidation of the unit.

The island has a relatively thin (up to 2 m) cover of Quaternary marine sediments overlying the Ordovician limestone bedrock.

Extensive shingle fields, consisting of limestone pebbles, have developed on the western coast of the island.

The nearest port is Dirhami 9 km to southeast, which is the starting point of most boat trips to the island.