This is written on the envelopes of various old-fashioned organisations, notably as part of the letterhead of the AMEB. Imagine my surprise the first time I received official mail from them. I could wish it said 'secret', but you can't have everything. It remains, however, a regular occupant of my mailbox.
Txikwa has very kindly added some legend to the story. Apparently some time in the C19th, a drunken mix-up led to the envelopes for HMSO, Her (or His) Majesty's Stationery Office were printed with OHMS instead. To save money, they simply pretended it was deliberate and made up something for it to stand for!