Wikipedia
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.
O.H.M.S. is a 1937 English adventure film directed by Raoul Walsh and starring Wallace Ford, John Mills, Anna Lee and Grace Bradley.