No longer accessible to road vehicles, Yarnton Lane still makes a great cycling and walking path connecting Kidlington and Yarnton. A footpath runs parallel to it for its entire length, alongside the fields, but to find this cache you'll need to be on the dirt track that's all that remains of the old road.
There are plenty of opportunities to switch from the footpath to the road and vice-versa, so if you'd rather take the footpath (recommended if it's wet as Yarnton Lane can get rather muddy!) then that works just as well.
You're looking for a "bison tube" style container about 5cm long.