Cheap, certainly. Also slow, and comfortable's in the eye of the beholder (I can't read in a road vehicle, so not for me). The Paddington services are much better these days, and there's also the excellent (new) Marylebone service. I live just outside Oxford and wouldn't dream of getting the bus.

My experience of road travel in the south was that delays were too frequent for journeys crossing the M25 for me to depend on it. That's from before good consumer traffic analysis apps; maybe those improve things enough.

Actually the Oxford to London bus services have been shutting down because of falling passenger numbers and traffic congestion.

https://www.bbc.co.uk/news/uk-england-oxfordshire-50992888

A lot of those passengers went over to Stagecoach's competing Oxford Tube service, which served a couple of different stops within London, but (in my experience) offered a much more frequent service.

There are nice, fast trains from Paddington to the West, including Oxford.