The 'Airbus' from Gloucester Green takes 80 mins to get to LHR central station or 100 mins to get to terminal 4. It goes every 30 minutes at most times of the day (I seem to remember) and it's a very good service. The driver is in contact with the preceeding bus and takes a different route if things are looking busy, so is rarely delayed. I cost about 17 Quid return when I last used it, probably gone up a bit now. I used to use it in preference to driving in sometimes when I used to work from LHR.
Getting to anywhere else from Oxford is a pain and usually involves rail.
www.oxfordbus.co.uk/heathrow.html for a timetable.