Obliteration: 1) A cancellation intended solely to deface a stamp-also called a killer. 2) An overprint intended to deface a portion of the design of a stamp, such as the face of a deposed ruler. A mark used to cancel stamp.
Obsolete: A stamp no longer available from post offices, although possibly still postally valid.
Occupation issue: An issue released for use in territory occupied by a foreign power.
Off-center: A stamp design that is not centered in relation to the edges of the stamp. Generally, off-center stamps are less desirable than stamps more nearly centered in relation to the edges. Stamps that are extremely off-center may be added to collections as production freaks.
Offices abroad: At various times, many nations have maintained post offices in other countries, usually because of the unreliability of the local postal system. In China and the Turkish Empire, especially, many foreign nations maintained their own postal systems as part of their extraterritorial powers. Usually, special stationery and stamps were used by these offices. Most consisted of overprints on the regular issues of the nations maintaining the offices.
Official: Stamp or stationery issued solely for the use of government departments and officials. In many countries such items may be available to collectors in unused condition from the postal authority.
Offset: 1) A printing process that transfers an inked image from a plate to a roller. The roller then applies the ink to paper. 2) The transfer of part of a stamp design or an overprint from one sheet to the back of another, before the ink has dried (also called set off). Such impressions are in reverse. They are different from stamps printed on both sides.
OG: The adhesive coating on a mint or unused stamp or envelope flap applied by a postal authority or security printer, usually before the item was issued. Upon request of stamp collectors, postal authorities have at times offered to add gum to items first issued ungummed.
OHMS: Abbreviation for On His (or Her) Majesty’s Service. Used in perfins, overprints or franks to indicate Official use in the British Commonwealth.
Omnibus issue: An issue released by several postal entities to celebrate a common theme. Omnibus issues may or may not share a keytype design.
On paper: Stamps (usually postally used) that are affixed to portions of original envelope or wrapper. Often used to describe stamps prior to soaking.
On piece: A stamp on a portion of the original envelope or wrapper showing all or most of the cancel. Stamps on piece are usually saved that way.
Original gum: The adhesive coating on a mint or unused stamp or envelope flap applied by a postal authority or security printer, usually before the item was issued. Upon request of stamp collectors, postal authorities have at times offered to add gum to items first issued ungummed.
Overprint: Any printing over the original completed design of a stamp. An overprint that changes the value of a stamp is also called a surcharge.
Oxidation: Darkening of the ink on certain stamps caused by contact with air or light. Some inks used to print stamps, especially oranges, may in time turn brown or black.