Anyone intending to travel to Oxford or the Chilterns by train from London between 3 Apri and 9 June needs to keep a close eye on the First Great Western website.
The line is closed due to issues with a viaduct near Abingdon (the Nuneham Viaduct over the River Thames), and currently does not have a specified re-opening date, but no sooner than 10 June.
Limited rail replacement bus services are in place and tickets are also being accepted on specified local bus services (see below).
Oxford passengers should travel via Chiltern from Marylebone.
This will also affect long distance trains from Reading to Birmingham and the South Coast run by Cross Country- these are now terminating/starting at Didcot and Oxford with a bus for the 35 minute journey (northbound)/40 minute journey (southbound) in between.
For clarity trains are running as scheduled from Oxford to Moreton-in-Marsh and on to Worcester.
Please note, from 11 April a road closure near to Oxford station will increase the journey time by approximately 10 to 15 minutes.
https://www.bbc.co.uk/news/uk-england-oxfordshire-65169011
EDIT 1 (4 April)- date extended to 23 April and Cross Country services clarified;
EDIT 2 (5 April)- use of Stagecoach routes S3 and S7 withdrawn, and note added about extended replacement bus journey times from 11 April (talk about bad timing). Until 7 April GWR timings are being confirmed the previous day at 1000, on 7 April this will change to a weekly update on Friday for the following week.