The church was first mentioned in the Domesday Book of 1086, where it was recorded that the owner of the land north of the city intended to build a Norman church there. The church was built for one Edwin, son of Godegose and finished in 1120. In 1139, Edwin granted the church and all its property to the then newly created Benedictine Godstow Abbey, 2 miles (3.2 km) to the northwest.
St Giles Church is 550 yards (500 m) north of Oxford's city wall, and when built it stood in open fields. There were no other buildings between it and the city wall, where the St Michael at the North Gate church stands. About a thousand people lived within the walls of Oxford at this time.
The church was not actually consecrated until 1200, by Saint Hugh, Bishop of Lincoln. There is a 13th or 14th century consecration cross consisting of interlaced circles cut into the western column of the bell tower that is believed to commemorate this. Also in commemoration of the consecration, St Giles' Fair was established. The fair continues to this day, held on the Monday and Tuesday after the Sunday following 1 September, which is St Giles' Day. St Hugh also expanded the St Mary Magdalen's Church to the south in 1194.