There is a cycle path that runs from the overpass at Wheatley all the way along the north edge of the A40 to Headington roundabout. It's not a brilliant surface, but it's not too bad (it's not that clear on google maps because it's quite close to the hedge). Then there's a pedestrian underpass to get you to the other side, where there's another cycle path to take you around to Summertown (that one involves a bit of quiet road). Not sure if there's a cycle path along the top edge of Oxford (but there is a small side road), and then there's another cycle path all the way out to Eynsham (on bot sides of the road I think - roadworks dependant).
You can hop off the cycle path quite quickly and go along the back roads through Forest Hill and Stanton St John, but you'll have to hook up to the A40 at some point.
PS I wouldn't want to ride the A40 along there. Too much traffic, travelling far too fast. A few years ago I would have thought nothing of riding the A34 from Oxford to Bicester, but I think the traffic is heavier and generally just less pleasant these days. Or maybe I'm just older!