Born in Quarrendon, Buckinghamshire (at that time part of Mercia), she was the neice of St Edith and St Edburga, and granddaughter of King Penda of Mercia (by his daughter Wilburga).

Raised in a convent (most likely at nearby Ascott) her ambition was to become a nun but she was forced into marriage by her father to King Sighere of Essex. Eventually she did establish a convent at Chich in Essex.

She was executed by Danish invaders at the holy waters in Quarrendon. Some say that the spring issued forth from the ground where she fell, though there is evidence of a holy spring at Quarrendon in the time of Osyth's aunts. Another legend tells of Osyth standing up after her execution, picking up her head and walking to the door of a local convent, before collapsing there.

Osyth's skeleton was stored for many years at St Mary's Church in Aylesbury where they were a site of great pilgrimage. However following a papal decree in 1500 the bones were removed from the church and buried in secret.