Where the author(s) name is not known (as is the instance with many dictionaries), use the title of the work (or web page) as your citation (if this is long you can use a shortened form).
Example in-text citation with University of Hull Harvard:
(Concise Oxford Dictionary, 2004)
Example in-text citation with University of Hull Footnotes: 1 Concise Oxford Dictionary (Oxford: Oxford University Press, 2004).