St Giles comprises a chancel with a S chapel, aisled nave, W tower and S porch. The 12thc church presumably consisted of nave and chancel, of which a portion of the nave walls survives above the 13thc arcade. Later in the 12thc at least the base of the existing W tower was built, and later again, clerestory windows were inserted into at least the N side of the nave, which must have by that time have had aisles. Early in the 13thc the nave, aisles and arcades were rebuilt, as well as the upper parts of the tower and the twin bell openings. The S chapel was added mid-13thc, and later in the 13thc the chancel itself was rebuilt.

The Romanesque features remaining are the blocked remains of two small round-headed clerestory window arches, visible over the N nave arcades. In the chancel S wall is an arched recess holding a 13thc sedilia that re-uses a pair of short Romanesque shafts with cushion capitals, one at each end.

St Giles was probably initially built as a private church by Edwin, son of Godegose, to whom Henry I confirmed it between 1123 and 1133 (VCH). It was built outside the city walls, as was the royal palace of Beaumont, built nearby to the SW in the 1130s. At the church dedication Walter, archdeacon of Oxford, granted it the tithes of his villeins at Walton, and it soon became parochial. In 1139 Edwin presented the church to Godstow nunnery who appropriated the rectory in 1221. It passed to the Crown at the Dissolution.

A column that supports the bell tower, near the organ, has been claimed as having a Romanesque capital (Barrington-Ward, 2012), but this seems very unlikely from its appearance.