Dorchester on Thames (or Dorchester-on-Thames) is a village and civil parish in Oxfordshire, England, about 3 miles (5 km) northwest of Wallingford and 8 miles (13 km) southeast of Oxford. The town is on the River Thame a few hundred yards from where it flows into the River Thames. The two rivers differ in spelling by one letter!
Historically the River Thames was only so named downstream of the village; upstream it is named the Isis, and Ordnance Survey maps (and some maps from other publishers) continue to label the river as "River Thames or Isis" above Dorchester. In practice, however, this distinction is rarely made outside the city of Oxford.
For more information, see the EN Wikipedia article Dorchester on Thames.