Given that Oxford station is not in the city centre
Its as good as - a ten minute walk at worst. You're right though there are pretty good road links north from the centre so buses would be the obvious mode of transport. There are two large-ish park & ride sites between Oxford and Kidlington which are both well served with buses which could travel on to/from the airport. They could also just about double as off-airport parking.
Incidentally the parkway station planned just north of Oxford as part of the Chiltern Line link-up is I believe going to be close to (maybe co-located with) one of these park & ride sites.
Does anyone have any info on the loads Minoan have been achieving?