Good the day when Óengus took Alba,
hilly Alba with its strong chiefs;
he brought battle to palisaded towns,
with feet, with hands, with broad shields
- The Book of Leinster
The second is the saltire in the clouds. The story of Scotland's flag - that a Scottish army beheld the Cross of St. Andrew in the sky before a battle against the Angles, a possible omen for their victory - is traditionally associated with Óengus' successor, Óengus II, and the legendary Angle king Æthelstan, and dates it to 832 AD. The problem with this date is that Æthelstan was not born until much later. However, The Scotichronicon suggests another Pictish King, one "Unust," and the Picts battle the Northumbrians rather than the Angles. Not only are the Northumbrians more suitable from a political and geographical point of view, "Unust" is not a million miles away from Onuist, the Pictish name of Óengus - and given the Son of Fergus' military achievements, it seems entirely possible that at least the tradition could date from the last year of his reign at the latest.
Most compelling of all, Óengus is generally credited with establishing a monastery to St. Andrew at Cennrígmonaid, modern St. Andrews: even if the story is ultimately folklore, it could be a thematic embellishment on the First King of the Picts & Scots' devotion to the apostle who would become the patron of all Scotland.