OAS - On Active Service.
OAT - Onward Air Transmission.
Obligatory tax stamp - A revenue stamp issued to collect funds for a national or philanthropic purpose. Its use was usually compulsory on letters in certain places and periods.
Occupation stamp - A stamp overprinted or specially printed for use in a conquered territory occupied by the victorious military forces.
Official letter - A formal communication in writing, generally exempt of postage or franked by special postage, used solely by designated 'official' departments of a governing body.
Official stamp - Stamp specially printed, or overprinted, for use solely by a designated 'official' department.
Official seal - A seal used to close mail opened or damaged by the postal service or accidentally left unsealed by the sender.
OHMS - On Her (His) Majesty's Service.
Overprint - Words or values printed on a stamp after it was completed. Overprints are used to provide stamps for countries which have none of their own; to define or alter the use of a stamp; to show changes in form of government; to mark a change in the name of a country; to denote occupation by a foreign force; to mark an anniversary.