[5][6] Surviving Pictish sources for the period are few, limited to king lists, the original of which was prepared in the early 720s,[7] and a number of accounts relating to the foundation of St Andrews, then called Cennrígmonaid.

[24] Once the dominant force in Britain, it remained a powerful kingdom, but the end of the old dynasty of kings with the death of Osric in 729 led to conflict between rival families for the throne.

[35] King Teudebur map Beli had ruled from Dumbarton Rock since 722, and continued to do so until his death in 752 when his son Dumnagual succeeded him.

[44] It is relatively nearby, at the hill of Moncrieffe, near Perth, that he first appears in the records, defeating his rival, Alpin (or Pictish Elphin), in battle.

[45] That the Irish annals envision his kin as "Eóganachta" suggests he was the descendant of an obscure "Vuen" (or Wen), the Pictish British cognate of Gaelic Éogan.

[57] In the 730s, Óengus fought against Dál Riata whose traditional overlords and protectors in Ireland, the Cenél Conaill,[58] were much weakened at this time.

[47][60] Dál Riata was ruled by Eochaid mac Echdach, possibly of the Cenél nGabráin[61] who died in 733, and the king lists are unclear as to who, if anyone, succeeded him as overking.

[62] The Cenél Loairn of north Argyll was ruled by Dúngal mac Selbaig whom Eochaid had deposed as overking of Dál Riata in 726.

[62] Dúngal, previously deposed as overking of Dál Riata, was overthrown as king of the Cenél Loairn[64] and replaced by his first cousin Muiredach mac Ainbcellaig.

[69] Muiredach of the Cenél Loairn was no more successful, defeated with heavy loss by Óengus's brother Talorgan mac Fergusa,[70] perhaps by Loch Awe.

There is the presence of Óengus's son Bridei at Tory Island, on the north-west coast of Donegal in 733, close to the lands of Áed Allán's enemy Flaithbertach mac Loingsig.

[47][79][note 8] In 740, a war between the Picts and the Northumbrians is reported, during which Æthelbald, King of Mercia, took advantage of the absence of Eadberht of Northumbria to ravage his lands,[83] and perhaps burn York.

Earnwine's father had probably been an exile in the north after his defeat in the civil war of 705–706,[86] and it may be that Óengus, or Æthelbald, or both, had tried to place him on the Northumbrian throne.

The battle of Catohic between the Britons and the Picts is reported at a place named Mocetauc (perhaps Mugdock near Milngavie)[92] in which Talorgan mac Fergusa, Óengus's brother, was killed.

[95][96] This is thought to refer to the coming to power of Áed Find, son of Eochaid mac Echdach, in all or part of Dál Riata, and his rejection of Óengus's overlordship.

[97][98] A number of interpretations have been offered of the relations between Óengus, Eadberht and Æthelbald in the period from 740 to 750, which due to the paucity of sources remain otherwise unclear.

This rests largely on a confused passage in Symeon of Durham's Historia Regum Anglorum, and it has more recently been suggested that the interpretation offered by Frank Stenton — that it is based on a textual error and that Óengus and Æthelbald were not associated in any sort of joint overlordship[102] — is the correct one.

[112] The Irish annals report the death of "Tuathalán, abbot of Cinrigh Móna", in 747,[113] making it certain that St Andrews had been founded before that date, probably by Óengus or by Nechtan son of Der-Ilei.

Óengus, like his successors and possible kinsmen Caustantín[123] and Eógan, is recorded prominently in the Liber Vitae Ecclesiae Dunelmensis, a list of some 3000 benefactors for whom prayers were said in religious institutions connected with Durham.

[127] His death is reported in the usual brief style by the annalists, except for the continuator of Bede in Northumbria, possibly relying upon a Dál Riata source, who wrote,[128][129][88] "Óengus, king of the Picts, died.

[142] Kings from his broader family continued to rule the Picts until they suffered a crushing defeat at the hands of Vikings in 839 when Óengus's great-grandson — and men "almost without number" — was killed.