Overlordship
Following the victory at Heavenfield, Oswald reunited Northumbria and re-established the Bernician supremacy which had been interrupted by Edwin. Bede says that Oswald held imperium for the eight years of his rule (both Bede and the Anglo-Saxon Chronicle say that Oswald's reign was actually considered to be nine years, the ninth year being accounted for by assigning to Oswald the year preceding his rule, "on account of the heathenism practised by those who had ruled that one year between him and Edwin"), and was the most powerful king in Britain. In the 9th-century Anglo-Saxon Chronicle he is referred to as a Bretwalda. Adomnán describes Oswald as "ordained by God as Emperor of all Britain".
He seems to have been widely recognized as overlord, although the extent of his authority is uncertain. Bede makes the claim that Oswald "brought under his dominion all the nations and provinces of Britain", which, as Bede notes, was divided by language between the English, Britons, Scots, and Picts; however, he seems to undermine his own claim when he mentions at another point in his history that it was Oswald's brother Oswiu who made tributary the Picts and Scots. An Irish source, the Annals of Tigernach, records that the Anglo-Saxons banded together against Oswald early in his reign; this may indicate an attempt to put an end to Oswald's overlordship south of the Humber, which presumably failed.
The Mercians, who participated in Edwin's defeat in 633, seem to have presented an obstacle to Oswald's authority south of the Humber, although it has been generally thought that Oswald dominated Mercia to some degree after Heavenfield. It may have been to appease Oswald that Penda had Eadfrith, a captured son of Edwin (and thus a dynastic rival of Oswald), killed, although it is also possible that Penda had his own motives for the killing.
Oswald apparently controlled the Kingdom of Lindsey, given the evidence of a story told by Bede regarding the moving of Oswald's bones to a monastery there; Bede says that the monks rejected the bones initially because Oswald had ruled over them as a foreign king. To the north, it may have been Oswald who conquered the Gododdin. Irish annals record the siege of Edinburgh, thought to have been the royal stronghold of the Gododdin, in 638, and this seems to mark the end of the kingdom; that this siege was undertaken by Oswald is suggested by the apparent control of the area by his brother Oswiu in the 650s.
Oswald seems to have been on good terms with the West Saxons: he stood as sponsor to the baptism of their king, Cynegils, and married Cynegils' daughter. Her name is reported by only one source, Reginald of Durham's 12th century Vita S. Oswaldi, which says that it was Kyneburga. Although Oswald had one known son, Æthelwald, it is uncertain whether this was a son from his marriage to Cynegils' daughter or from an earlier relationship—since Æthelwald began ruling in Deira in 651, it has been argued that a son from this marriage would have been too young at the time to be trusted with this position, and therefore may have been older, the product of a relationship Oswald had during his exile.
Read more about this topic: Oswald Of Northumbria