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.
Surviving 12th century features of the church include two windows in the north side of the clerestorey of the nave and the lower parts of the bell tower. The tower was finished early in the 13th century, which is the date of the aisle arcades and Early English Gothic lancet windows as well. The Decorated Gothic chancel was built late in the 13th century.