O. C., on Center - The measurement of spacing for studs, rafters, joists, and the like in a building from the center of one member to the center of the next.

O. G., or Ogee - A molding with a profile in the form of a letter S; having the outline of a reversed curve.

Oakum - Loose hemp or jute fiber that's impregnated with tar or pitch and used to caulk large seams or for packing plumbing pipe joints.

Open Hole Inspection - When an engineer (or municipal inspector) inspects the open excavation and examines the earth to determine the type of foundation (caisson, footer, wall on ground, etc.) that should be installed in the hole.

Oriented Strand Board or OSB - A manufactured 4' X 8' wood panel made out of 1"- 2" wood chips and glue. Often used as a substitute for plywood.

Outrigger - An extension of a rafter beyond the wall line. Usually a smaller member nailed to a larger rafter to form a cornice or roof overhang.