Nigel has mentioned a couple of current disruptions, but there are some more that are on 'popular' routes here.
In particular, the line from Didcot Parkway and Oxford is closed between 30 July and 14 August. This is to raise the level of the line by about half a metre and install new culverts to prevent the flooding that has become all to regular at Hinksey, just south of Oxford.
So coming from London Paddington for Oxford you would need to change to a bus at Didcot. People travelling to places in the north of Oxford may find it more convenient to go to Oxford Parkway from London Marylebone and take the bus in from there, although this may encounter the roadwork delay.
Note that there are additional closures:
30 July to 7 August, between Oxford and Banbury; the railway will be closed for signalling improvements
3 to 7 August, between Oxford and Hanborough; the railway will be closed for bridge repairs (affects travellers through to Cotswold stations)