Open valley -  Method of valley construction in which shingles on both sides of the valley are trimmed along a chalk line snapped on each side of the valley. Shingles do not extend across the valley. Valley flashing is exposed.

Organic felt - An asphalt roofing base material manufactured from cellulose fibers.

Overhang - That portion of the roof structure that extends beyond the exterior walls of a building.

O.C. - On Center. A measurement term meaning a certain distance between like materials. Studs placed at 16" O.C. will be laid out so that there is 16" from the center of one stud to the center of the next.

OHMMETER – In electrical contracting, a device to measure the resistance across a load. They are never used on a live circuit. It is used to track down broken wires.

OHM'S LAW - States that, in a given electrical circuit, the amount at current in amps is equal to the pressure in volts divided by the resistance in ohms. The formula is:

I (Current) = V voltage or V = I x R

R resistance or R = V/I

OIL-CANNING - The term describing distortion of thin-gauge metal panels which are fastened in a manner restricting normal thermal movement.

ORGANIC - A term designating any chemical compound which contains carbon and hydrogen.

OVERHANG - That part of the roof structure which extends horizontally beyond the vertical plane of the exterior walls of a building.

OXIDIZE - To combine with oxygen in the air.

OIL-CANNING - The term describing distortion of thin-gauge metal panels which are fastened in a manner restricting normal thermal movement.

ORGANIC - A term designating any chemical compound which contains carbon and hydrogen.

OVERHANG - That part of the roof structure which extends horizontally beyond the vertical plane of the exterior walls of a building.

OXIDIZE - To combine with oxygen in the air.