the cheapest airport to fly into and the one with the best transport links to Oxford
These are not the same. Heathrow has the best transport links to Oxford, with a frequent coach service - every half hour for most of the day - which takes 90-100mins from the central bus station, rather quicker from T5. The 737 Oxford-High Wycombe-Luton Airport-Stansted Airport (full timetable most easily read here: www.transportforbucks.net/Uploads/Files/737.pdf ) runs 8 times a day, including the wee hours, so has a service frequency of about 2-4 hours depending upon the time of day, and takes a little over 2 hours from Luton Airport to Oxford. So the question is, will you be reasonably confident of getting the specific coach you are aiming for, because if you miss it the next one might be a long wait. If you just miss one, then I suppose you can travel via London, but it's not much fun. Aer Lingus flights to Heathrow are sometimes not much more expensive than Ryanair to Luton, and a small premium might be worthwhile for the more convenient access. But certainly Luton is rather better than Stansted for Oxford. Up to you.