He, along with Bóand, helped Óengus search for his love.

Dúngal later returned to Scotland, and was captured in 736, along with his brother Feredach, by Óengus.

In 744 the Picts acted alone, and in 750 Óengus may have cooperated with Eadberht of Northumbria in a campaign in which Talorgan, brother of Óengus, was killed in a heavy Pictish defeat at the hands of Teudebur of Alt Clut, perhaps at Mugdock, near Milngavie.

Æthelbald, who might have been allied with Óengus, the king of the Picts, took advantage of Eadberht's absence from Northumbria to ravage his lands, and perhaps burn York.

Óengus runs Cellach through with his " dread spear ", which has three chains attached to it ; these chains wound one of Cormac's advisers and blind Cormac in one eye.

Eventually it lands on the clothes of Óengus, who recognises it as Étaín, but he is at war with Midir and cannot return her to him.

The legend surrounding Scotland's association with the Saint Andrew's Cross was related by Walter Bower and George Buchanan, who claimed that the flag originated in a 9th century battle, where Óengus II led a combined force of Picts and Scots to victory over the Angles, led by Æthelstan.

The Flag of Scotland shows the Saltire of Saint Andrew and is traditionally associated with Óengus mac Fergusa.

Previously thought to have been of Dál Riatan origin and descended from Fergus mac Echdach, their family is now assumed to have been that of the first king Óengus mac Fergusa, perhaps originating in Circinn ( presumed to correspond with the modern Mearns ), a Pictish family with ties to the Eóganachta of Munster in Ireland.

The later St Andrews tradition recounting the supposed arrival of Saint Regulus ( or Saint Rule ) at St Andrews, with relics of St Andrew, has him met at Forteviot by three sons of Óengus: Eogán, Nechtan and Finguine.

According to legend, Achaius, King of Scots ( possibly coming to the aid of Óengus mac Fergusa, King of the Picts ), while engaged in battle at Athelstaneford with the Saxon King Æthelstan of East Anglia, saw in the heavens the cross of St Andrew.

Later scribes added a prose preface, including material on Óengus, and accompanied the text with abundant glosses and scholia.

These pagan seats of power are contrasted with the great ecclesiastical centres of Ireland which were flourishing in Óengus ' own time, such as Armagh and Clonmacnoise.

" Not satisfied with his hermitage, which was only a mile from Clonenagh, and, therefore, liable to be disturbed by students or wayfarers, Óengus removed to a more solitary abode eight miles distant.

Mongfind also occurs as the name of the Pictish princess wedded to Conall Corc, ancestress of the Eóganacht Locha Léin of Iarmuman and of a line of powerful Pictish kings, e. g. Óengus I of the Picts, but it appears this can be dismissed as a case of coincidental influences on the story associating Crimthann mac Fidaig with the Pictish king and father, called Feredach.

Óengus is said to have fathered Fíacha on his own daughter when drunk, and to have put him in a boat, wrapped in a purple robe with a golden fringe and accompanied by treasure, and set him out to sea – hence the epithet fer mara, " man of the sea ".

It may be that Óengus was involved in wars in Ireland, perhaps fighting with Áed Allán, or against him as an ally of Cathal mac Finguine.

A defeat here for Eadberht and Óengus by Æthelbald's Mercians would correspond with the claim in the Saint Andrews foundation legends that a king named Óengus son of Fergus founded the church there as a thanksgiving to Saint Andrew for saving him after a defeat in Mercia.

Later generations may have conflated this king Óengus with the 9th century king of the same name.

Óengus, like his successors and possible kinsmen Caustantín 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.

The following 9th century Irish praise poem from the Book of Leinster is associated with Óengus:

Óengus succeeded his brother Caustantín to the throne.

It is likely that his reign ended as a result of his defeat at the hands of the Picts of Fortriu, led by Talorgan mac Fergusa, brother of Óengus, at " Cnoc Cairpri in Calathros at Etarlinde ".

The 750 battle between the Britons and the Picts is reported at a place named Mocetauc ( perhaps Mugdock, near Milngavie ) in which Talorgan the brother of Óengus was killed.

Constantín's family dominated Fortriu after 789 and perhaps, if Constantín was a kinsman of Óengus I of the Picts ( Óengus son of Fergus ), from around 730.

Other Gaels, such as Caustantín and Óengus, the sons of Fergus, were identified among the Pictish king lists, as were Angles such as Talorcen son of Eanfrith, and Britons such as Bridei son of Beli.

Lulach's son Máel Snechtai was Mormaer of Moray, while Óengus of Moray was the son of Lulach's daughter.

In order to hide their affair, Dagda made the sun stand still for nine months ; therefore their son, Óengus, was conceived, gestated and born in one day.

Teudebur died around 752, and it was probably his son Dumnagual who faced a joint effort by Óengus and Eadberht in 756.

He was the son of King Caustantín and succeeded his uncle, Óengus, to the throne.

In 733 the Annals of Ulster report that Dúngal profaned Tory Island by forcibly removing Bruide, who is presumed to be the son of Óengus mac Fergusa, king of the Picts.

Some have proposed that Wimund was a son of Óengus of Moray ( died 1130 ), grandson of King Lulach mac Gille Coemgáin.

He ruled for 21 years, fighting many battles against the descendants of Érimón, before he was killed by Fíachu's son Óengus Olmucaid in the battle of Clíu.

Óengus Olmucaid ( or Aengus Olmucada ), son of Fíachu Labrainne, was, according to medieval Irish legend and historical tradition, a High King of Ireland.

* Rothechtaid mac Main, son of Maen, son of Óengus Olmucaid, according to medieval Irish legend and historical tradition, High King of Ireland

Óengus Ollom (), according to medieval Irish legend and historical tradition, was a High King of Ireland who was the son of Ailill, the son of Labraid Loingsech.

He took power after killing his predecessor, Óengus Ollom, and ruled for seven or ten years, until he was killed in Ulster by Fer Corb, son of Mug Corb.

Fergus Fortamail (" strong, prevailing "), son of Bresal Brecc, son of Óengus Gailian, son of Ailill Bracan, son of Labraid Loingsech, was, according to medieval Irish legend and historical tradition, a High King of Ireland.

He took power after killing the previous incumbent, Eochaid Ailtlethan, in battle, and ruled for either eleven, twelve or twelve and a half years, until he was killed by Eochaid's son Óengus Tuirmech Temrach in battle at Tara.

Óengus Tuirmech Temrach, son of Eochaid Ailtlethan, was, according to medieval Irish legend and historical tradition, a High King of Ireland.

Óengus died in 834, the only event of his reign reported in the Irish annals, and was succeeded by his nephew Drest mac Caustantín.