Traveling by plane
The recommended airports are Luton (40km), Heathrow (90km), Birmingham International (UK, not US!, 99km).
Other airport options apply, but are not as convenient: London City (112km), Stansted (120km), Gatwick (147km).
Traveling to meetings in Oxford by plane
Heathrow (45 min by car, 60 by bus) and Gatwick (2h by car, 2.5 by bus). Other airport options apply, but are not as convenient: Birmingham International (UK, not US!), Luton, London City, Stansted, in that order.
From Heathrow and Gatwick, you hitch the airline bus to Oxford:
http://airline.oxfordbus.co.uk/ (about £30 for a return ticket).
Traveling by train
Book your https://www.thetrainline.com/ ticket to Milton Keynes or Oxford.
Traveling by coach
From London: hop on the National Express to Milton Keynes Coachway or hop on the Oxford Tube.