Find your way onto the Oxford Canal (you could park at Port Meadow pay and display then walk back over the railway brigde and down onto the canal from the ramp near William Lucy Way), then keep an eye out for the entrance to the Trap grounds.
There are direct and indirect paths to the cache. I suggest the indirect ones, as then you will get to experience the trap grounds in all their glory. The 'Emperor Moth' is especially worth seeking out.
Some info from the website:
The Trap Grounds consists of three acres of reed bed (a rare fragment of a type of wetland habitat once common around Oxford) and four acres of scrubland – formerly waste-ground, and now a rich mosaic of wildlife habitats. The name (in recorded use since at least 1832) may derive from the practice of trapping birds here … or the making of eel traps from willow withies … or the parking of pony-drawn traps during the eighteenth-century races on Port Meadow … or the dumping of night-soil from the ‘traps’ or privies of university colleges. Or it may be a corruption of the designation ‘Extra Parochial’, which denoted the site’s exemption from the payment of church tithes.
The cache is a 35mm film canister with just a log roll inside. Bring your own pen or pencil.