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).

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.

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. Also called surprint.

Over Run
Additional printed matter beyond order. Overage policy varies in the printing industry. Advance questions avoid blind knowledge.