Offer       A proposal outlining terms and conditions for the purchase of a property that may be accepted, rejected, or countered by the seller. If accepted, the offer becomes a contract.

One-on-One       The application of a single ply of roofing over the substrate, followed by the
application of a second single ply over the first (phased application).

On-grade       Pertaining to the location of a finished floor with no portion below ground, and with the floor and the ground in contact or separated by less than 18 inches of well-ventilated space.

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.

Option       The right to purchase a property under certain terms and conditions, usually for a specific price and during a specific period of time.

Organic       Composed of hydrocarbons or their derivatives, or of plant or animal matter.

Origination fee       A fee charged by a lending institution for processing the paperwork on a loan. This is in addition to any points which the lender may charge. See also processing fee.

Oriented Strand Board (OSB)       A structural panel made of wood strands sliced in the long direction and bonded together with a binder under heat and pressure.

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

Overlay Shingle       A one-piece base shingle to which overlay pads, consisting of an additional layer of asphalt and granules, are applied in random patterns to simulate two-piece laminated shingles.