King Olaf Trygvason's Saga, best known Icelandic kings' saga written in Old Norse ca. 1230) that the Oeselians and the Curonians (from neighbouring Curland) were called Víkingr frá Esthland - Vikings from Estonia. Another hypothesis suggests that the name Saaremaa was used for the whole archipelago of islands. The main island was called Kurresaar, thus Curonians might have simply been another name for Oeselians.
Source: Wikimedia commons.
The Livonian rhymed chronicle tells that, "The Oeselians, neighbors to the Kurs (Curonians), are surrounded by the sea and never fear strong armies as their strength is in their ships. In summers when they can travel across the sea they oppress the surrounding lands by raiding both Christians and pagans."
Scandinavian sagas mention several raids of the Swedish, Norwegian and Danish Vikings on Saaremaa.
"In the end of the 10th century there was a battle between the Norwegian jarl Erik, who was ravaging the coasts of Saaremaa, and a four-ship group of Danish Vikings sailing in the same waters. In the Njall saga there is a description of a battle that took place in 972 AD between Icelandic and Estonian Vikings. The battle was fought somewhere near the northern coast of Saaremaa and was won by the Icelanders.
About 1008 AD Olaf the Holy, who later became the king of Norway, landed on Saaremaa. The Osilians, taken by surprise, had at first agreed to pay the tax he demanded but then gathered an army at the time of the negotiations and attacked the Norwegians. Olaf nevertheless won the battle. Around the year 1030 a Swedish Viking chief called Fröger was killed in a battle on Saaremaa.
The most detailed description of Viking Age Estonia can be found in the saga of the young Norwegian prince Olaf Trygvasson written by Snorre Sturlusson. In 967 when Olaf was three years old, he was travelling with his mother Estrid and many companions to Novgorod. On the way, Osilian pirates attacked the ship. Both Olaf and his mother were taken prisoners and sold into slavery. Olaf was resold to different owners many times before he was bought on a market by his uncle and thus regained freedom. Years later, Estrid was also freed." (source: www.saaremaa.ee)
In Gesta Danorum by Saxo Grammaticus one can find that "Olimar, that renowned tamer of savage peoples, vanquished Thorias the Tall, king of the Jämts and Hälsings, with two other leaders just as powerful, not to mention also Estland, Kurland, Oland and the islands that fringe the Swedish coast."