Oistin mac Amlaíb (Old Norse: Eysteinn Óláfsson) was a ninth-century Norse or Norse-Gael leader whom sometimes identified as a King of Dublin. He was a son of Amlaíb Conung and nephew of Ímar, founder of the Uí Ímair dynasty. He is sometimes identified with Thorstein the Red, a figure who features in the Norse sagas.^{[citation needed]}

The earliest mention of Oistin may be by Cogad Gáedel re Gallaib, which in 873 describes how Bárid mac Ímair and an unnamed son of Amlaíb Conung plundered the caves of Ciarraige.^{[nb 1][1]} This unnamed son of Amlaíb is probably Oistin.^[2] Downham suggests this raid undertook as a show of strength; it occurred shortly after the death of Ímar, with Bárid probably succeeding him as King of Dublin.^[3] Though Oistin is not named as a king in the annals, it has been suggested by others that Oistin and Bárid ruled together as co-kings after the death of Ímar.^[4]

The one and only mention of Oistin by name in contemporary annals is by the Annals of Ulster in 875:^[5]

Oistín son of Amlaíb, king of the Norsemen, was deceitfully killed by Albann.^{[nb 2][7]}

"Albann" is generally agreed to be identical to Halfdan Ragnarsson, supposed son of the legendary Viking Ragnar Lodbrok.^{[nb 3][9]} Some scholars identify Halfdan as a brother of Amlaíb, Ímar, and Auisle, and hence uncle of Oistin.^[10] This identification is contingent upon Ímar being identical to Ivar the Boneless: Halfdan and Ivar are named as brothers in the Anglo-Saxon Chronicle.^{[11][nb 4][nb 5]} If Halfdan and Oistin were indeed kin, then it may explain the reason for the conflict: it was a dynastic squabble for control of the kingdom.^[13]

Some have suggested Oistin is the same person as Thorstein the Red, a figure who features in the Norse sagas.^[14] However, this is problematic since Thorstein is said to have died in Scotland after conquering much of the country, whereas Oistin is said to have been killed in Ireland in 875 by Albann (i.e. Halfdan).^[15] Similarly, Oistin's father Amlaíb is sometimes identified with Olaf the White, a Viking sea-king who also features in the sagas and is named as the father of Thorstein the Red.^[16] The sagas are of dubious historical value, but the figures featured within may be based on real people.^[17] To get around the problem whereby Oistin and Thorstein die in different places in different circumstances, but keeping the identification of Amlaíb with Olaf the White, it has been proposed that Oistin and Thorstein were brothers.^[18]

Amlaíb Conung is identified as Oistin's father by the Annals of Ulster.^[19] A brother, Carlus, is mentioned by the Annals of the Four Masters.^[20]

A number of other familial connections are known if the identification of Oistin with Thorstein the red is correct. According to the sagas, Aud the Deep-Minded, daughter of Ketil Flatnose, the King of Isles, was the mother of Thorstein.^[21] Thorstein's wife is given as Thurid (Þuríður in Old Norse), the daughter of a Geatish man named Eyvind the Easterner. Their children include at least six daughters, Osk, Vigdis, Olof, Thorhild and Thorgerd, and at least one son, Olaf Feilan.^[22]

• Thorpe, B, ed. (1861). The Anglo-Saxon Chronicle. Rerum Britannicarum Medii Ævi Scriptores. Vol. 1. London: Longman, Green, Longman, and Roberts. Accessed via Internet Archive.
• "Annals of the Four Masters". Corpus of Electronic Texts (16 December 2013 ed.). University College Cork. 2013. Retrieved 23 November 2014.
• "The Annals of Ulster". Corpus of Electronic Texts (15 August 2012 ed.). University College Cork. 2012. Retrieved 23 November 2014.
• Ellwood, Thomas, ed. (1898). The Book of the Settlement of Iceland. Kendal: T. Wilson. Accessed via Internet Archive.
• Todd, JH, ed. (1867). Cogad Gaedel re Gallaib: The War of the Gaedhil with the Gaill. London: Longmans, Green, Reader, and Dyer. Accessed via Internet Archive.
• Jones, Gwyn (1999). Eirik the Red and other Icelandic Sagas. Oxford University Press. ISBN 978-0-1995-3915-4.
• Magnusson, Magnus; Palsson, Hermann (1969). Laxdaela Saga. Penguin Classics. ISBN 978-0-1404-4218-2.
• Cook, Robert (2002). Njal's Saga. Penguin Classics. ISBN 978-0-1404-4769-9.
• Thorsson, Örnólfur; Scudder, Bernard (2005). The Saga of Grettir the Strong. Penguin Classics. ISBN 978-0-1404-4773-6.

• Byock, Jesse L. (1990). Medieval Iceland: Society, Sagas, and Power. University of California Press. ISBN 978-0-520-06954-1.
• Costambeys, Marios (2004). "Hálfdan (d. 877)". Oxford Dictionary of National Biography. Oxford University Press. doi:10.1093/ref:odnb/49260. Retrieved 20 December 2014. Subscription or UK public library membership required.
• Downham, Clare (2007). Viking Kings of Britain and Ireland: The Dynasty of Ívarr to A.D. 1014. Edinburgh: Dunedin Academic Press. ISBN 978-1-903765-89-0.
• Hudson, Benjamin T. (2004). "Óláf the White (fl. 853-871)". Oxford Dictionary of National Biography. Oxford University Press. doi:10.1093/ref:odnb/49263. Retrieved 5 December 2014. Subscription or UK public library membership required.
• Ní Mhaonaigh, Máire (1996). "Cogad Gáedel re Gallaib and the annals: a comparison". Ériu. 47: 101–126. JSTOR 30007439.
• Royal Philosophical Society of Glasgow (1913). Proceedings of the Royal Philosophical Society of Glasgow, Volumes 44–45.
• Sigurðsson, Jón Viðar; Bolton, Timothy, eds. (29 November 2013). Celtic-Norse Relationships in the Irish Sea in the Middle Ages 800-1200. Brill. ISBN 978-90-04-25512-8.
• South, Ted Johnson (2002). Historia de Sancto Cuthberto. Boydell & Brewer. ISBN 978-0-85991-627-1.
• Woolf, Alex (2007). From Pictland to Alba: 789 - 1070. Edinburgh University Press. ISBN 978-0-7486-1234-5.

• CELT: Corpus of Electronic Texts at University College Cork. The Corpus of Electronic Texts includes the Annals of Ulster and the Four Masters, the Chronicon Scotorum and the Book of Leinster as well as Genealogies, and various Saints' Lives. Most are translated into English, or translations are in progress.