Low floor bus

https://www.gwr.com/travelling-with-us/replacement-buses

All buses we use, whether they are coaches, local buses or taxis, will be wheelchair accessible.

Coaches have a lift, enabling wheelchair users to access the vehicle. All local buses we use will be low-floor vehicles with ramp access.

Due to the layout of some stations, it may not be possible to use the lift. In these situations an accessible taxi will be provided instead.

for low-floor vs coach think city bus vs Greyhound.

Ah! my imagination was picturing something a bit more basic. That sounds just fine, and now I know bus vs coach. Thank you!

A low-floor bus being one you don't have to take several steps up when you board. You would have just the step from the pavement into the bus, but no steps once you are in the bus.

Chiltern Railways go to Oxford from Marylebone
Have you checked out their services?
I would expect a low floor bus to go lower on its suspension at a stop so it is level with the foot path. Some have extending ramps too to bridge any gap

There are a whole set of different bus routes covering the closure. My guess is that the local stopping services between Didcot and Oxford, and Oxford and Banbury will most likely be buses, and the more limited stop services of Didcot-Oxford-Banbury, Didcot-Hanborough, and Oxford-Hanborough most likely to be coaches. The non-stop Didcot-Oxford could reasonably be either.

However, I see that they are warning that because of the general shortage of bus drivers the services might not be as frequent as timetabled.

low floor buses are excellent in many ways, and many buses are also able to "kneel".

One teeny tiny problem with them is that because the floor is between the wheel arches instead of on top the walking area can become more constricted passing the wheels and the seats on top of the wheel arches involve a pretty good hoick up. The rest are ok though.