I think it's just the Oxford Airline bus sold another way. At Heathrow T2 or T3, follow the signs towards the underground, they're the most common signs point you the right way. When you're basically outside the entrance to the London Underground, you'll see lifts and escalators sign posted as the Central Bus Station. Head up and there's a big departures board showing all the buses. The Oxford bus normally leaves from stop 15. See https://www.theairlineoxford.co.uk/o...-heathrow-bus/ for more info on the bus, and https://www.theairlineoxford.co.uk/c...owards-oxford/ for live tracking of the coaches.

If you're booking with megabus make sure you're at the stop nice and early, they're tend to be super helpful for people on their own tickets, but I think they can be pretty strict on using the right bus if you've bough a cheap megabus ticket. If the saving isn't much, just book your ticket direct with Airline Bus and take any coach