Osthryth was not the first of her family to become a Mercian queen. Her sister Alhflæd had married Peada, King of South Mercia 654-656. After Peada's death, allegedly murdered with Alhflæd's connivance, she retreated to Fladbury in Worcestershire, to judge from the place-name, which means "stronghold of Flæde", and its subsequent history. Sometime in the 690s Æthelred granted Fladbury to Oftfor, Bishop of Worcester, to re-establish monastic life there.S76 However this grant was later contested. Æthelheard, son of Oshere, maintained that Æthelred had no right to give Fladbury away, as it had been the property of Osthryth. Æthelheard claimed it as her kinsman and heir.
Æthelred and Osthryth loved and favoured Bardney Abbey in Lincolnshire. Osthryth placed there the bones of her uncle Oswald of Northumbria, who was venerated as a saint. It is clear from this story that Osthryth played a part in promoting the cult of St Oswald. Many years later she persuaded Oswald's widow Cyneburh to take the veil.
Osthryth had to contend with major conflicts of loyalty. In 679 her brother Ecgfrith of Northumbria fought a battle against Æthelred, in which Ecgfrith's brother Ælfwine was killed. Bede tells us that he was "a young man of about eighteen years of age and much beloved in both kingdoms, for King Æthelred had married his sister."
The murder of Osthryth in 697 by Mercian nobles is unexplained in the sources that mention it. Ann Williams attributes it to the hostility between the Mercians and the Northumbrians, while D. P. Kirby suggests that it may have been revenge for her sister's alleged involvement in Peada's murder. Finberg speculates that she and her kinsman Oshere were suspected of trying to detach the kingdom of the Hwicce from Mercian overlordship. She was buried at Bardney Abbey.
- Kelly, F. E. (2004). "Osthryth (d. 697)". Oxford Dictionary of National Biography. Oxford University Press. doi:10.1093/ref:odnb/20909. Retrieved 20 May 2012. (subscription or UK public library membership required)
- Bede, The Ecclesiastical History of the English People (1994), 144.
- Hooke, in The Anglo-Saxon Landscape: The Kingdom of the Hwicce (1985), p. 11, argues that Fladbury could have belonged to another sister of Osthryth, Ælflæda, Abbess of Whitby, but she had no known connection with Mercia and died in 713, after Osthryth, so Osthryth could not have inherited Fladbury from her.
- H.P.R.Finberg, The Early Charters of the West Midlands (Leicester 1961), p.170.
- Bede, The Ecclesiastical History of the English People ed. J. McClure and R. Collins (1994), 126.
- H.P.R.Finberg, The Early Charters of the West Midlands (Leicester 1961), p.165.
- Bede, The Ecclesiastical History of the English People ed. J. McClure and R. Collins (1994), 207.
- Bede, The Ecclesiastical History of the English People ed. J. McClure and R. Collins (1994), 292; Anglo-Saxon Chronicle.
- Ann Williams, 'Osthryth', in Ann Williams et al, eds, A Biographical Dictionary of Dark Age Britain, Seaby, 1991
- D. P. Kirby, The Earliest English Kings, Routledge, 1991, p. 127
- H.P.R.Finberg, The Early Charters of the West Midlands (Leicester 1961), pp. 176-7.