St Berin's mission to the West Saxons in 634 led to the baptism of King Cynegils in the River Thames, close to where Dorchester Abbey now stands. Cynegils gave Dorchester to be an episcopal See.
In 664, the See was moved to Winchester, Dorchester having become a bishopric in the Kingdom of Mercia. The seat of the bishops remained at Dorchester until the Norman Conquest.
Following the dissolution of the monasteries, Henry VIII created six Sees, including Oxford. In 1547, Edward VI granted additional rectories and advowsons to the bishopric. King Charles I granted Bishop Bancroft and his successors a pension of £100 per annum and a licence to unite the vicarage of Cuddesdon to the rectory. Bishop Bancroft built a palace at Cuddesdon, which was burned down in the Rebellion and rebuilt by Bishop Fell after the Restoration.
In 1836, the county of Berkshire and parts of Wiltshire were annexed to the Diocese of Oxford, and in 1837, the county of Buckingham similarly, although this annexation did not take effect until 1845.