Offset Printing 

Printing technique that transfers ink from a plate to a blanket to paper instead of directly from plate to paper. 
 
Opacity 
(1) Characteristic of paper or other substrate that prevents printing on one side from showing through the other side. 
(2) Characteristic of ink that prevents the substrate from showing through. 
 
Onion Skin
A specific lightweight type (kind) of paper usually used in the past for air mail. Seldom used today (in the typewriter era). 
 
Opaque 
(1) Not transparent. 
(2) To cover flaws in negative with tape or opaquing paint is also called as block out and spot. 
 
Open Prepress Interface 
Hardware and software that link desktop publishing systems with color electronic prepress systems. 
 
Outer form 
Form (side of a press sheet) containing images for the first and last pages of the folded signature (its outside pages) as compared to inner form. 
 
Outline Halftone 
Halftone in which background has been removed or replaced to isolate or silhouette the main image is also called as knockout halftone and silhouette halftone. 
 
Overlay 
Layer of material taped to a mechanical, photo or proof. Acetate overlays are used to separate colors by having some type or art on them instead of on the mounting board. Tissue overlays are used to carry instructions about the underlying copy and to protect the base art. 
 
Overlay Proof 
Color proof consisting of polyester sheets laid on top of each other with their image in register, as compared to integral proof. Each sheet represents the image to be printed in one color. Also called celluloid proof and layered proof. 
 
Overprint 
To print one image over a previously printed image, such as printing type over a screen tint is also called as surprint. 
 
Over Run 
Additional printed matter beyond order. Overage policy varies in the printing industry. Advance questions avoid blind knowledge. 
 
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