I'd stay to the north of Oxford and pick up the A44 there, the A44 is a bit quieter than the A40 and a bit twistier. Fish Hill is a great set of sweepers if you get a clear run at them and worth it just for that. There's a roundabout at the bottom in case you want to go back up for another go...
Depends how long you've got but A40, A44 (Fish Hill is shelgripped for max lean), A49 then A51 would be my choice for an A-Road route.