Not to be pedantic, but I believe that iPagan is wondering about the 20 volume Oxford English Dictionary, not the single-volume Oxford Dictionary of English. 😎
The former is not, to my knowledge, available for iDevices at this time. The latter, is, and I use the excellent ODEDeluxe, which also includes their thesaurus. The interface is excellent, and updates are regular. The interface has not yet been updated for native iPad use, but they claim it will be coming soon.