Harwell is a large village in the NE part of the traditional county, 2 miles W of Didcot. The church, in the village centre, has an aisled nave, W tower and transepts of c.1190-1220, chancel c.1300, nave aisles heightened c.1300. Remains of a 11thc. nave were excavated to W of present church in 1962-63. The stiff-leaf and crocket capitals of the nave are not treated here, but the stylistically earlier exterior capitals of the W tower are. There is also a plain font.

In 1074 a dwelling at Harwell and certain tithes were granted to the Chapel of St George in Oxford Castle, which implies the presence of a chapel at that date. In 1149 these and other tithes were transferred to the Augustinian abbey at Oseney, Oxfordshire. Harwell comprised three holdings at DS, one held by the Bishop of Winchester and two by Roger of Ivry. The larger of the latter (called the Upper Manor) had a chapel, presumably the one mentioned above. The Upper Manor had previously been granted to Robert d'Oilly by the Conqueror and he gave it to Roger of Ivry. When the Ivry line became extinct c.1157 the Honour of Beckley, to which the manor now belonged, passed to the family of St Valery. In 1231 the Honour passed to Henry III's brother Richard, Earl of Cornwall in whose family it remained until 1359-61 when the Black Prince granted the advowson of the church and the Upper Manor to the College of St Nicholas in Wallingford Castle. They reverted to the Crown when the college was suppressed in 1548.

The Harwell church guide finds connections between the present church and the design of Oseney Abbey which held the tithes (see above). The font bowl may have come from an earlier building, though not necessarily; the chamfered base certainly belongs with the c.1200 campaign. Despite the tower capital being stylistically earlier than those of the nave there is no reason to assume that they were carved at an earlier date. It may well be that their simpler old-fashioned forms were considered more suitable for decoration which could only be seen from a distance.