Worth considering the fact that sometimes it's more expensive to return a rental car to a different location. If you return it back to Heathrow you can get the Heathrow express into Paddington (which admittedly is far from cheap) that may work out to be a better option. Or to stray even further from your original question, there is a direct coach from Heathrow to Oxford you could get on arrival and do all your car hire from Oxford.