The Oeselians or Osilians are a historical subdivision of Estonians inhabiting Saaremaa (Latin: Oesel or Osilia), an Estonian island in the Baltic Sea.

In modern Estonian, they are called saarlased ("islanders"; singular: saarlane), which also applies to present-day inhabitants of the island.

They are first thought to be mentioned as early as the 2nd century BCE in Ptolemy's Geography III.The Oeselians are known in the Old Norse Icelandic Sagas and in Heimskringla as Víkingr frá Esthland (English: vikings from Estonia).

Their sailing vessels are called pirate ships by Henry of Livonia in his Latin chronicles from the beginning of the 13th century.

Eistland or Esthland is the historical Germanic language name that refers to the country at the eastern shores of the Baltic Sea in general, and is the origin of the modern national name for Estonia.

The mainland of modern Estonia in the 8th century Ynglinga saga is called Adalsyssla in contrast to Eysyssel or Ösyssla that is the name of the island (Swedish: Ösel, Estonian: Saaremaa), the home of the Oeselians.

In the 11th century, Courland and Estland (Estonia) are both denoted separately by Adam of Bremen.On the eve of Northern Crusades, the Oeselians are summarized in the Livonian Rhymed Chronicle thus: "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."