Buses from Heathrow, Luton, Gatwick and Stansted to Oxford (Gloucester Green) are operated by National Express (website). Tickets may be purchased in advance, or on the bus.
Additional buses from Heathrow and Gatwick to Oxford (Gloucester Green) are also operated by The Airline (website).
Trains from Birmingham International to Oxford Station are operated by Cross Country (website).
Ride share apps such as Uber, as well as taxis are also available.
Other options require multiple trains and buses, and are therefore not highly recommended.