transportation for Heathrow to Oxford

It looks like a good option would be the National Express bus - departures every 10-20 min. during peak hours, cost: £24. http://www.nationalexpress.com/home.aspx

We travel to Oxford at least once a year and always use the Airline bus, which operates 24/7. They depart Heathrow about every 20 minutes during the day, less frequently at night and tickets may be purchased on the bus using pounds or dollars. By sure to purchasing a return (roundtrip) ticket, the return cost to Heathrow almost northing. If memory serves me correctly, it does make a couple of stops before terminating at Gloucester Green, the main bus station in central Oxford, about 1 1/2 hours after departing Heathrow. Just enough time for a quick nap. From there, it's an easy walk to anywhere in town.

National Express act as resellers for the Oxford Bus Company's "the airline" (sic) service. There are actually 8 drop off stops in Oxford but I guess most services won't have to stop at all of them.

We'll be using "the airline" bus ("coach" in British) to Oxford in three weeks. It was easy to make the reservation and pay for the tickets online. While we have reserved seats on the bus leaving about two hours after we land at Terminal 5, the tickets are good for any one within 24 hours before or after, in case of flight changes or delays. We'll be getting off at the High Street stop, a little short of the main Oxford bus station at Gloucester Green, because it's closer to our hotel. My research led me to think this is the best combination of speed, convenience, and price for this transfer. I'll try to let you know if I was wrong.

We took the airline bus and it worked well for us. Thought we might get a cab on arrival at the bus depot in Oxford but never saw one so just walked to lodging. That might be an issue if door to door service is desired.

There will likely be plenty of cabs at the train station.

The taxi rank for the bus station is behind the bus station arrivals on the other side of Gloucester Green on St Georges Place. (Gloucester Green despite the name is today a paved square.)