Oxford train and bus/coach stations are about a 10-15 minute walk or £5-10 taxi from the college.
There is NO and I mean NO parking in Central Oxford or at the college. Anyone driving will have to leave their cars in one of the park and ride car parks on the edge of Oxford.
http://www.oxford.gov.uk/transport/car-parking.cfm
There is a direct Oxford Coach link for Heathrow.
My recommendation: come by train.