From Banbury, walkers on the Oxford Canal towpath will pass Grant's Lock on the way to Twyford Wharf. It is a pleasant place to stop for a rest and gongoozle, next to bridge number 174.
You're looking for a small container near the lock and bridge area, but away from the water.
The cache can only be reached from the towpath and easily reached from Banbury or from the canal bridge on Twyford Road where there is parking. There is also a footpath from the Oxford Road, south of Banbury down to Nadkey Bridge as an alternative. Cyclists should be aware there are sometimes a few thorns on the ground after hedge-trimming.