At least from Oxford, there's a choice of ways to get to London; I currently use the X90 coach service, which is slower stop-to-stop (OXF->PAD is timetabled at 60 minutes, while the X90 typically takes 80 minutes between the stops I use), but it's more reliable, and I'd lose the time getting from Headington to OXF anyway.

Sucks if you're not able to use other choices, of course 🙁