Oxford to Exeter is a tricky one. A34 is the worst road I use regularly - although the junction with the A303 is a hoot if you can get a free run at it. 
Oxford to Exeter is probably quickest A40/M5, assuming you don't do it on a Bank Holiday Friday afternoon, as you'll shagged at Birdlip... ...but get delayed so badly, all the traffic around Avonmouth will have cleared

Oxford to Exeter itself, A420/M4/M5 is usually far quicker nowadays than A34, although you can get stuck behind the tin tent brigade thats frustrating (and on the dual carriageway sections, you'll always get a knobjockey pull out and then continue at the same speed as what they are overtaking.
My trouble is to get to Oxford involves a particularly shitty bit of the A34, so I occasionally go M40/M42/M5 to Exeter... ...every time I am daft enough to try A34/A303 I regret it, as it doubles the journey length. Last time, it was 8.5hrs from Brackley to Penzance. Crazy.