"Officials" are stamps used for post between government departments.
There were four issues of definitives overprinted with O.H.H.S (On His Highness' Service) in 1907, 1914, 1915 and 1922; and one issue overprinted O.H.E.M.S (On His Egyptian Majesty's Service) in 1922-23, in English and the Arabic equivalent.
Most interesting are the variations that exist in the way the Arabic characters are written. Some of these variations are so rare that such stamps go for £££. Unfortunately, I don't have any of the rare ones :¬( but I still like the fact that these are yet another type of "Egyptian Earlies" which fit nicely into my collection.
Here's an example of a variation. See how the Arabic characters are written differently?
At the moment, there's only one O.H.E.M.S overprint: