Obligatory tax stamp - a revenue stamp issued to collect funds for national or philanthropic purpose
Occupation stamp - a stamp overprinted or specially printed for use in a conquered territory occupied by the victorious military forces
OCR - an acronym for 'Optical Character Reader'. In the Postal Service, Optical Character Readers are used to read mail addresses and spray corresponding bar codes onto the envelopes for processing. The first postal OCR was used in the Detroit post office in 1965.
Official issue - a stamp issued for use on official correspondence. When inscribed or overprinted for use by specific government departments, the stamp is referred to as 'Departmental'.
Official seals - a seal used to close mail opened or damaged by the postal service or accidentally left unsealed by the sender
Offset printing - a printing process often used with lithography in which an inked image is transferred from the plate to a roller. In an intermediate step, the roller applies ink to the paper. The design is transferred from the plate to a rubber blanket, and then re-transferred to the stamp paper.
On-the-Fly - an exchange of mail between Railway Post Office cars and post offices without stopping the train. Mail was exchanged via cranes on which postmasters hooked a mail pouch. As the RPO car passed by, the mail was snagged by a hook attached to the side of the train car. Mail bound for the post office was placed in a pouch that was then tossed off the train and retrieved by the postmaster.
Overprints - words or devices 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; or to identify valid issues when identical stamps are in alien hands.