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O.H.M.S., an initialism for On Her Majesty's Service or On His Majesty's Service (depending on the sex of the reigning monarch), is an official franking commonly seen on correspondence from government departments in the United Kingdom, Canada and other countries that share the same monarch as their head of state (now known as Commonwealth realms).
In Canada, O.H.M.S may be written on an envelope instead of using postage stamps for any letters being sent to the Canadian House of Commons or to any member of the Canadian Parliament. Letters sent from Lieutenant-Governors may also bear the O.H.M.S. mark.
In popular culture
O.H.M.S. is a 1937 action comedy film, while the title of Ian Fleming's 1963 James Bond novel On Her Majesty's Secret Service, along with its film adaptation, is a play on the term.