State of Oman stamps were issued by the separatist Government of the Imamate of Oman in exile. The rebels started issuing stamps in 1967 and set up offices in friendly Arab capitals, including Cairo, Beirut and Baghdad and lobbied for international recognition. Most Arab countries recognized them as the legitimate Omani government.
Since the rebels had long left Oman and did not have any postal services there, the letters with the stamps were dispatched from Baghdad (January 1968 only), then from Amman (until 1971) and later from Damascus (until June 1972). This means that the issues from 1967 to 1972 had postal validity in the above mentioned places. 
Clive Feigenbaum, a partner in London, continued to print new stamps for State of Oman (and also for Dhufar, a province of today's Sultanate of Oman) until 1986. These Feigenbaum issues, of course, did not see any postal use and are considered as Fantasy stamps or Bogus. They were done in sheetlets of 8 different se-tenant stamps, same as Feigenbaum did for Staffa, Nagaland, Eynhallow, ISO, etc.

The Regular Camel set was released in 1967.
Have followed different overprints:
in 1968 the Space Achievement – Apollo 8 set,
the overprint of Green Mountain in 1969,
and in 1970 the Kennedy Memorial.
Also, transport Day, unknown date.

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Member: The Royal Philatelic Society of Canada
The Christmas Seal & Charity Stamp Society
Haiti Philatelic Society
The British North America Philatelic Society