Continuing the discussion from New Postage Stamps next year:
That’s an interesting question concerning an arrangement that I’ve never heard of but Wiki says:
In Canada, the initialism O.H.M.S. may be written on an envelope instead of using postage stamps for any letters being sent to the House of Commons of Canada or to any member of the Parliament of Canada. Letters sent from lieutenant-governors may also bear the O.H.M.S. mark. The French translation Service de sa Majesté (SDSM) is also used.
In Australia before Federation, letters in envelopes franked with the O.H.M.S. mark were exempt from postal fees.
From Hansard:
Mr. Joseph Slater
The inscription “O.H.M.S.” has no postal significance and its appearance on letters does not in any way relieve senders of the general necessity to pay postage on all correspondence. Government Departments and some closely associated bodies, but not the nationalised industries, are allowed to use envelopes printed or franked with an “Official Paid” design: but they still have to pay postage on such letters.
Mr. Joseph Slater
The inscription O.H.M.S. has no postal significance. Postage stamps must be affixed to envelopes unless by arrangement they bear a design showing that postage has been paid. This arrangement does not extend to nationalised industries.
So, presumably, “O.H.M.S.” postal privilege in the UK was terminated before 1965 - perhaps another FM will know exactly when.