Osmund was a King of Sussex

Kingdom of Sussex

The Kingdom of Sussex or Kingdom of the South Saxons was a Saxon colony and later independent kingdom of the Saxons, on the south coast of England. Its boundaries coincided in general with those of the earlier kingdom of the Regnenses and the later county of Sussex. A large part of its territory...

, apparently reigning jointly with Oswald

Oswald of Sussex

Oswald was an Ealdorman of Sussex, jointly with three former kings: Osmund, Ælfwald, and Oslac.He witnessed a charter of Offa, King of Mercia, dated 772 as Osuualdus dux Suðsax . He was listed ahead of the three former kings Osmund, Ælfwald, and Oslac, which implies that his rank was higher. It is...

, Ælfwald

Aelfwald of Sussex

Ælfwald was a King of Sussex, who reigned jointly with Ealdwulf and Oslac, and probably also with Oswald and Osmund.Ælfwald witnessed an undated charter of Ealdwulf, believed to be from about 765, with his name corruptly recorded as Ælhuuald rex....

, and Oslac

Oslac of Sussex

Oslac was a King of Sussex. He reigned jointly with Ealdwulf and Ælfwald, and probably also Oswald and Osmund.Oslac witnessed an undated charter of Ealdwulf, believed to be from about 765, with his name corruptly recorded in the surviving revision as Osiai rex .After the conquest of Sussex by Offa,...

.

According to the Anglo-Saxon Chronicle

Anglo-Saxon Chronicle

The Anglo-Saxon Chronicle is a collection of annals in Old English chronicling the history of the Anglo-Saxons. The original manuscript of the Chronicle was created late in the 9th century, probably in Wessex, during the reign of Alfred the Great...

, version D, Osmund was reigning in Sussex when Archbishop Cuðberht died in 760, so his rule commenced before that event.

He issued a charter, dated 762 in error for 765, as Osmundus.

He also issued a charter dated 770 in which he is listed as Osmundus rex.

So Osmund’s reign was from in or before 760 to between 770 and 772, as he witnessed a charter of Offa

Offa of Mercia

Offa was the King of Mercia from 757 until his death in July 796. The son of Thingfrith and a descendant of Eowa, Offa came to the throne after a period of civil war following the assassination of Æthelbald after defeating the other claimant Beornred. In the early years of Offa's reign it is likely...

, King of Mercia, dated 772 as Osmund dux. Evidently he was demoted from king to ealdorman following Offa's conquest of Sussex.