More details are now available of this total line closure-
https://www.gwr.com/travel-information/travel-updates/planned-engineering/oxford
Briefly Cross Country will not run between Leamington Spa and Didcot.
Chiltern will not run between Oxford Parkway and Oxford.
Great Western will not run between Didcot, Oxford and Banbury, also between Oxford and Hanborough on the line to Moreton in Marsh for the Cotswolds and Worcester.
On the Moreton in Marsh line a revised timetable will be in force.
All timetables are yet to be announced.
Bus/coach replacement services will be in place over all affected route sections.
As this is planned work it is unlikely that train tickets will be accepted on local bus services, as happened during the earlier bridge closure south of Oxford, because enough rail replacement coaches will have been sourced. Nor is it likely that train tickets to Moreton in Marsh will be accepted via any alternative all rail route this time.
Passengers for Oxford should consider using the Oxford Tube coach service from London instead.
This will also all apply on 13 August, 10 September, 16 September and 15 October